The Utility of the Personal Wellbeing Index Intellectual Disability Scale in an Australian Sample

Transcription

1 Journal of Applied Research in Intellectual Disabilities 2009, 22, The Utility of the Personal Wellbeing Index Intellectual Disability Scale in an Australian Sample J. A. McGillivray*, A. L. D. Lau, R. A. Cummins* and G. Davey à *School of Psychology, Deakin University, Victoria, Melbourne, Australia; Department of Rehabilitation Sciences, Hong Kong Polytechnic University, Hong Kong; à Psychology Department, University of Chester, Chester, UK Accepted for publication 29 September 2008 Background Subjective wellbeing (SWB) in people with intellectual disabilities has been the focus of increased interest in the identification of support needs and as an outcome measure for interventions and service delivery evaluations. It is therefore important to conduct further research in this area, and to develop appropriate scales to measure SWB. Methods A new scale, the Personal Wellbeing Index- Intellectual Disability (PWI-ID) was administered to 114 adults with mild (n = 82) or moderate (n = 32) level ID in Victoria, Australia. Results The PWI-ID demonstrated good reliability and validity. A comparison of the findings with previous research indicates that participants SWB levels are within the normative range, and are similar to those reported by the general population. Conclusions The results support the notion that individuals with ID do not experience life quality lower than normal, which can be explained theoretically by the Theory of Subjective Wellbeing Homeostasis. The use of the PWI-ID may ultimately assist in ensuring that the needs of people with ID are being met and inform the planning and delivery of congruent resources and services. Keywords: intellectual disability, PWI-ID scale, subjective wellbeing Introduction There is growing interest in the measurement of life quality at the level of populations, sub-groups and individuals. As Schalock (2000, 2004) reminds us, there is little that is more important to measure as an index of the human condition and it comes in two flavours objective and subjective. The former is simple to measure and forms the basis of life quality judged by such traditional criteria as medical health and wealth. Subjective wellbeing (SWB) is more difficult to measure reliably and validly, most particularly in people with intellectual disabilities (ID). However, it is increasingly being used as an outcome measure for interventions and service delivery evaluations because the perspective of the person with a disability is now considered to be a key variable in identifying their support needs and assessing the value of the support they receive. It is therefore unfortunate that none of the available measurement instruments have adequately demonstrated psychometric properties (see Cummins 2005 for a review). This highlights the necessity to conduct further research into the development of appropriate scales to measure SWB, most particularly amongst individuals with ID. Research about SWB in the field of ID appears to have been hindered because it is not easily measurable. Difficulties have included scale construction and administration, and challenges associated with obtaining selfreport from this population. Proxy responding, which involves the provision of responses by another person (e.g. a relative, friend, the investigator or another individual) on behalf of the person with ID, is occasionally adopted to overcome the respondent s difficulty in meeting the cognitive demands of a data gathering instrument. However, although proxy concordance can be achieved, there are serious concerns about its reliability and validity, and whether it accurately reflects the perceptions of the individual concerned (Budd et al. 1981; Stancliffe 1999; Cummins 2002a; Perry & Felce 2003; Perkins 2007). A number of scales have been Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Ltd /j x

2 Journal of Applied Research in Intellectual Disabilities 277 developed, such as the Lifestyle Satisfaction Scale (Heal & Chadsey-Rusch 1985) and the Quality of Life Scale (Schalock & Keith 1993), and, more recently, the Personal Outcome Measures of the Council on Quality and Leadership (2007) and the eight-domain Model of Quality of Life (Verdugo et al. 2007). However, all these are limited because they cannot be norm-referenced back to the general population. Likewise, most scales designed for the general population are not appropriate due to their lack of sensitivity to the unique needs of a respondent with ID (Finlay & Lyons 2001). It is therefore advantageous to develop a scale that is suitable for administration across subsections of the population, thus enabling comparison of SWB levels and their determinants and permitting the application of the same theoretical principles. These unresolved issues led to the development of the Personal Wellbeing Index (PWI) to provide a valid measure of SWB, alongside the creation of a collaborative and international network of scholars to develop and use it (Lau et al. 2005; International Wellbeing Group 2006). SWB has been traditionally measured by a single question regarding how people rate their satisfaction with life as a whole (Diener 1984). However, there is now recognition that the life satisfaction construct can be represented in a number of life domains (Scollon et al. 2005). The PWI scale contains items that ask people how satisfied they are with eight life domains: standard of living, personal health, achievement in life, personal relationships, personal safety, community-connectedness, future security, and religion spirituality. These domains represent the first-level deconstruction of life as a whole and provide insights into the various aspects that shape SWB (Cummins 1996, 1997; International Wellbeing Group 2006). Answers are reported on an 11-point Likert type scale with anchor points of completely satisfied (10), neutral (5), and completely dissatisfied (0). The PWI is now being used by a growing community in more than 48 countries, and has been translated and administered in various languages, including Algerian, Arabic, Chinese (Cantonese, Mandarin, Tibetan), Croatian, Dutch, English, Italian, Japanese, Mexican, Norwegian, Persian, Russian, Slovakian, and Spanish (International Wellbeing Group 2006). The PWI is an improved version an earlier widelyused scale, the Comprehensive Quality of Life Scale (Com- Qol), which received generally favourable appraisal, and was rated as having satisfactory to excellent validity, reliability, sensitivity, and utility (Hagerty et al. 2001). It had also been used successfully to measure the SWB of people with ID (Cummins 1995). However, it was later revised because it measured SWB through the creation of a multiplicative composite, in which domain importance was multiplied by domain satisfaction (Cummins 2002a); there were also issues pertaining to the scale s conceptual structure, particularly in relation to objective QOL. The PWI scale was developed to simplify its conceptual structure and factorability by measuring only SWB; to change the response scale to avoid psychometric confusion; and to optimize respondent discriminative capacity. For a detailed explanation and justification of these changes, see Cummins & Gullone (2000) and Cummins (2002b). The PWI is very similar in structure to the original subjective ComQol sub-scale. The psychometric performance of each is also much the same. The satisfaction scores from all domains are simply summed to produce a mean satisfaction value, known as the PWI score, to represent SWB. The PWI is also unique because it is embedded in the Australian Unity Wellbeing Index, which has been used since 2001 in regular quarterly surveys of the Australian general population to measure how satisfied people are with their lives. The scale has demonstrated extraordinary stability over time, and continues to prove to be a valid, reliable and sensitive instrument (Cummins et al. 2007). In addition, Cronbach a lies between 0.7 and 0.8, and the seven domains of the scale form a single stable factor that predicts over 50% of the variance in satisfaction with life as a whole. The extensive use of the scale as part of the Australian Unity Wellbeing Index provides data from the general population that can be used to norm-reference findings obtained from sub-populations, such as people with ID. A clear advantage of the PWI scale is that while it has been used predominantly in the general population, there is a parallel version (PWI-ID) designed for people who have ID or other forms of cognitive impairment. To date, however, no studies have described or demonstrated the use of the PWI-ID. Therefore, the first aim of this paper is to introduce and describe the PWI-ID as an appropriate tool for the measurement of SWB in people with ID. It is unique amongst SWB scales because, in addition to the availability of parallel versions, it includes a well structured pre-testing protocol to determine whether, and to what level of complexity, respondents with mild or moderate ID are able to use it. This ensures that individuals whose response to scales is unlikely to be valid, such as those with severe or profound ID, are excluded from the sample (Sigelman et al. 1981b; Chadsey-Rusch et al. 1992; Petry & Maes 2006). The comprehensive pre-testing protocol includes initial

3 278 Journal of Applied Research in Intellectual Disabilities screening of potential respondents for acquiescent responding, and a test for response scale competence. Another feature of the PWI-ID, in comparison to its equivalent for the general population, is its simple and concretely worded questions, and the option of substituting numerical response scales with a reduced choice format illustrated as a series of outline faces (from very happy to sad) to enhance comprehension (March 1992; Heal & Sigelman 1995). Further, reduced-choice formats (5-, 3-, and 2-point) are available for those who are not able to use the standard 11-point scale. These modifications are important as careful attention to the development of survey items, especially response format and wording, can greatly enhance self-report completion (Perkins 2007). There is no published study that validates the use of the PWI-ID scale. Thus, the second aim of this study is to investigate the utility of the PWI-ID within an Australian sample of people with ID. On the basis of the previous successful use of the PWI in the general population, as well as the ComQol amongst people with ID (Cummins 1995; Cummins et al. 1997), it is predicted that the PWI-ID will be an appropriate measure of SWB in people with ID. Another topic that requires exploration is the normative SWB level of people with ID. As pointed out by Cummins (1997), there appears to be a prevailing notion that people who have disabilities will have a life quality lower than that recorded in non-disabled populations. While there is some evidence to support this view (Ahlsiö et al. 1984; Viitanen et al. 1988), there is also support for the idea that people who experience a major life event that disrupts their life satisfaction levels have a strong tendency to return to their previous levels over time, and that groups with and without an ID report similar life satisfaction (Heady & Wearing 1992; Cummins 1995; Cummins et al. 1997; Pretty et al. 2002). These contrasting perspectives are difficult to evaluate because the scales used previously to explore the SWB of the general population differ from those used with ID samples. The PWI-ID will help to shed light on this issue. Working from the premise that most people seem to be happy and satisfied with their lives when their basic physical and social needs are met (Cummins 1995, 1998), Cummins postulated the Theory of Subjective Wellbeing Homeostasis (Cummins & Nistico 2002; Cummins et al. 2002; Cummins & Lau 2004). This theoretical framework posits that a person s SWB is normally maintained within a narrow positive set-pointrange by an active set of psychological devices and personality factors. Under normal living conditions, momentary fluctuations in SWB are caused by changing life experiences. However, cognitive adaptations act to return SWB back to with its normal range. Under other conditions, however, adverse factors can cause a homeostatic defeat that drives and maintains SWB below the normal range (Tiliouine et al. 2006). Thus, according to this theory, SWB does not vary freely over the range, but is held fairly constant in a manner similar to other homeostatically maintained processes, such as blood pressure or body temperature. For Western populations, the normative range for population sample mean scores is points on a scale distribution, with a mean of 75 (Lau et al. 2005) and within Australia this range is just three percentage points, from 73.4 to 76.4 points (Cummins et al. 2007). Thus, as a consequence of Wellbeing Homeostasis, it is predicted in this study that the majority of people with ID will experience a normal (positive) level of SWB, despite facing challenges associated with their disability and sometimes difficult living conditions. In summary, the main thrust of this study is to introduce the PWI-ID, exemplify its use, explore its psychometric properties, and to provide insights into normative SWB levels amongst people with ID. The specific objectives are to: 1 Provide psychometric data on the PWI-ID; 2 Determine the reliability and validity of the PWI-ID scale compared with the regular adult version of the PWI; 3 Compare the SWB levels of people with ID with preexisting normative data on the general adult population; and 4 Compare the SWB of people with mild and moderate ID. The use of the PWI-ID may ultimately assist in ensuring that the needs of people with ID are being met and inform the planning and delivery of congruent resources and services. Method Participants Convenience sampling was used to recruit 114 people with mild (82) or moderate (32) level ID in Victoria, Australia. They were recruited from government and non-government organizations and agencies that provide services or programs for people with ID, such as sheltered workshops and supported-employment settings.

4 Journal of Applied Research in Intellectual Disabilities 279 An invitation letter and consent form was sent to the organizations, who then contacted potential participants and their guardians, if applicable, to seek their interest and informed consent. The selection of participants was based on the following criteria: (i) formal diagnosis of mild (IQ: 50 69) or moderate (IQ: 35 49) ID; (ii) within the age range of years; (iii) able to follow simple verbal instructions and offer verbal responses; (iv) emotionally and medically stable; and (v) no history of mental illness or autism. Table 1 summarizes the gender, age, and socioeconomic characteristics (education, income, and marital status) of the sample. Of the 114 participants, 62 (54%) were male and 52 (46%) were female. Half of the sample was in the age category years, and the mean age was 33 years. Almost all participants were unmarried. With regard to education, 57 (50%) had received special education, 34 (30%) had received secondary school education, and 7 (6%) had primary education. As shown in Table 1, the majority of the sample lived with their parents (52; 46%), siblings (5; 4%), or alone (13; 11%). Measures and procedure Face-to-face individual interviews were conducted in English by a trained interviewer at a location preferred by each participant; 76 respondents were interviewed at their work setting; 14 at their home; and 21 at another venue such as rehabilitation centres and day activity centres. Each interview lasted approximately 45 min, and caregivers agency staff were not present during the testing. The pre-testing protocol and contents of the PWI-ID scale were administered according to the test manual (Cummins & Lau 2005). After an initial screen to identify and exclude participants who could not cope with the test demands due to level of disability, the pre-test involved questions to screen for acquiescent responding and a test of response scale competency. For people unable to comprehend the standard 11-point response scale, a set of reduced-choice format scales (5-, 3-, and 2-point) were provided. These use drawings of faces (from very happy to very sad) in the place of numbered scale choice points (see instruments/pwi/id-manual.pdf). Following successful completion of the pre-test, eligible participants responded to questions on Happiness with life as a whole and the PWI-ID scale items, at the level of choice format determined by the maximum level of discriminative competence displayed in the Table 1 Demographic and socioeconomic characteristics of the sample Respondents n % Male Female Mild Moderate Age and above 3 3 Unreported 2 2 Marital status Never married Married 6 5 De facto living together 4 4 Separated divorced 6 5 Widowed 1 1 Education attainment No formal education 2 2 Primary 7 6 Secondary Junior High Matriculation Senior High 9 8 Tertiary 2 2 Special Education Others (don t know) 9 8 Employment status Unemployed Sheltered Workshop Vocational Training Open Employment Voluntary Work 8 7 Others (don t know) 6 5 Living situation Alone Parents Siblings 5 4 Spouse partners Children 2 2 Other adults 4 4 Supervised group home Supervised hostel 7 6 Others (carer) 6 5 pre-testing. In order to identify any confounding factors that could have affected their self-reporting, the participants were asked if they had experienced a recent event which had a strong influence on their wellbeing.

5 280 Journal of Applied Research in Intellectual Disabilities In order to determine test retest reliability, the PWI-ID was re-administered to 31 randomly selected participants (representative, in demographic and socioeconomic characteristics, of the 114 group) at 1 2 weeks following its initial administration. Prior to analysis, the dataset was checked (for normality, skew, and kurtosis) to confirm that it satisfied assumptions for parametric tests. Descriptive statistics were used to summarize data on demographic variables and the PWI-ID. Cronbach s alpha, and itemtotal and inter-domain correlations were performed to examine the internal reliability of the scale. An intraclass correlation coefficient (one-way random model) and kappa values evaluated test retest reliabilities. The structure of the PWI-ID was explored via exploratory factor analysis, and bivariate correlations and multiple regression analyses probed the inter-relationships between domains and their contribution to life as a whole. Analysis of Variance (anova) was employed to compare group comparisons such as mild and moderate ID. Results Because individuals with ID are particularly prone to acquiescent responding, the data were checked prior to analysis for response sets, i.e. whether a respondent scores at the top or bottom of the scale consistently for all domains. In the present study, a total of 37 data sets from individual respondents were eliminated prior to analysis because they showed maximum scores for all seven items. Satisfaction ratings of the PWI-ID Table 2 summarizes the means and standard deviations of the PWI-ID domain ratings, which range from to The PWI-ID score of falls slightly above the normative range of the Australian population ( ). The mean domain ratings and the PWI-ID score were similar to those previously identified by Lau et al. (2005) in the general population of Victoria, Australia, in which the PWI score was 78.8 and the domains ranged from 75.3 to A statistical comparison (unequal variance t-test) of the mean ratings of the sample in the present study and the previous Victorian general population study (Lau et al. 2005) shows that the PWI-ID and PWI scores did not differ significantly, although there were differences between three domains: standard of living (t = 3.60, d.f. = 293, P = ), health (t = 2.17, d.f. = 293, P = 0.03), and feeling part of the community (t = 3.33, d.f. = 293, P = 0.001; Table 2). Further comparison with general population data throughout Australia (Cummins et al. 2007), reveals no significant differences, except for life achievement (t = 2.3, d.f. = 2000, P = 0.02). The personal relationships and feeling part of the community domains of the PWI-ID had the highest means, followed by life achievement, personal safety, standard of living, future security, and health. Four domains (life achievement, personal relationships, personal safety, and feeling part of the community) were above the PWI-ID mean score. In comparison, Lau et al. (2005) found that standard of living, personal relationships, and personal safety mean item scores were above the index mean score. Table 2 PWI-ID ratings compared with general population samples PWI-ID PWI 1 PWI 2 Items M SD M SD t P M SD t P Satisfaction with: Standard of living Health Life achievement Personal relationships Personal safety Feeling part of the community Future security Personal Wellbeing Index Satisfaction with life as a whole PWI scores for the general population in Victoria, Australia (n = 180; taken from Lau et al. 2005). 2 PWI scores for the general population in Australian (n = 2001; taken from Cummins et al. 2007).

6 Journal of Applied Research in Intellectual Disabilities 281 There was no significant gender difference in PWI and domains scores (see Table 3). There was no age difference, except for the life achievement domain, in which the younger group (18 30) had a significantly higher rating (76.70) than the older group (51 65; 49.50). An advantage of the PWI-ID scale is that it offers reduced choice format scales for respondents with different levels of cognitive impairment. In the present study, 72 people were able to use the 11-point scale, 20 used the 5-point scale, 19 used the 3-point scale, and 3 used the 2-point scale. Respondents who used the 3-, 5-, or 11-point scale did not report statistically different PWI-ID scores (78.57, 76.10, and respectively) or domain scores. However, they did report a statistically different score of their satisfaction with life as a whole ; specifically, the 3-point scale differed from the 5- and 11-point scales (Table 4). (Note: although the 2-point Table 3 PWI-ID ratings of different genders and age groups Male Female M1 SD M2 SD M1 M2 1 M SD M SD M SD M SD Satisfaction with: Standard of living ) Health Life achievement ) * * Personal relationships ) Personal safety Community connectedness Future security ) Personal Wellbeing Index ) Satisfaction with life as a whole ) There was no significant difference between the domain and PWI-ID scores of male and female respondents. 2 The only age difference was the scores reported for life achievement, which differed significantly between the and age groups. *P < Table 4 Respondents ratings using the 2-, 3-, 5- or 11-point version of the PWI-ID; and comparison of mild and moderate groups 2-point n =3 3-point n =19 5-point n =20 11-point n =72 Mild ID n =82 Moderate ID n =32 PWI-ID Domains M SD M SD M SD M SD M1 SD M2 SD M1 M2 2 Satisfaction with: Standard of living * Health * Life achievement Personal relationships Personal safety Community connectedness )0.05 Future security Personal Wellbeing Index (ID) Satisfaction with life as a whole * ) The only difference on the basis of choice format was on life as a whole scores, where 3-point format differed significantly from the 11- and 5-point versions. 2 Mild and moderate ID groups differed significantly on standard of living and health domains. *P < 0.05.

7 282 Journal of Applied Research in Intellectual Disabilities scale resulted in a substantially smaller PWI-ID score, the sample is too small to consider it statistically). The PWI-ID scores of respondent groups with mild and moderate ID were also compared (see Table 4). There was generally no appreciable difference in scores between the two groups, although two domain scores (standard of living and health) were significantly higher the mild ID group. Internal reliability The Cronbach a coefficient is 0.76, which demonstrates acceptable internal reliability, and is comparable to previous research of the PWI in the Australian general population (0.80; Lau et al. 2005). The item-total (domain-pwi) correlations ranged between 0.37 and Compared to previous research, the item total correlations in the present study are higher. The domain inter-correlations ranged between 0.16 and 0.54 (Table 5). The highest correlations were between community connectedness and personal relationships (0.54); standard of living and life achievement (0.49); and personal relationships and future security (0.45). To examine test retest reliability of the scale, it was re-administered to 31 randomly selected participants (representative, in demographic and socioeconomic characteristics, of the 114 group) at 1 2 weeks following its initial administration. The analysis revealed an intraclass correlation coefficient of 0.57, and test retest coefficient of Table 6 Factor analysis of the PWI items Rotated component matrix(a) Component 1 2 Personal relationships 0.75 Future security 0.75 Health 0.66 Community connectedness 0.64 Standard of living 0.85 Life achievement 0.81 Personal safety Eigenvalues % of variance that all the variables inter-correlate with at least one other variable at >0.48, which confirms the suitability of the data for factor analysis. The factorability of the correlation matrix also met the other assumptions, including a Kaiser Meyer Olkin measure of 0.803, and a statistically significant Bartlett s test of sphericity (P < 0.001). Table 6 shows the extraction of two factors that explain about 57.97% of the variance, which is higher than that found by Lau et al. (2005; 40.76). However, one of the items is complex, with loadings on both factors. Therefore, since three items is considered the minimum number for a factor, this analysis resolves to a single factor, in line with the previous literature. Validity With the aim of determining the coherence of the scale, the domains were subjected to a principal components analysis, followed by a varimax rotation (with Kaiser Normalization). The correlation matrix (Table 6) shows Table 5 Domain inter-item correlations Domain Standard of living Health 0.16 Life achievement Personal relationships Personal safety Community connectedness Future security Shared and unique contribution of individual domains to Life as a Whole All domains of the PWI-ID correlate significantly with life as a whole, and ranged between 0.27 and 0.44 (see Table 7). To determine the unique contribution of the domain to life as whole, the former were regressed against the latter. Within the PWI-ID, Personal relationships and Safety constitute the largest unique contribution to the prediction of life as a whole, although all domains were statistically significant contributors (adjusted R 2 = 0.27). This is a much lower level of explained variance than is typically found in general population samples. Discussion The focus of this paper is to introduce the PWI-ID as a reliable and valid tool to measure the SWB of people with mild and moderate levels of ID. The scale probes

8 Journal of Applied Research in Intellectual Disabilities 283 Table 7 Regression of personal domains on satisfaction with life as a whole Variable Life as a wholer B b sr 2 Standard of living 0.27* Health 0.34* Life achievement 0.26* Personal relationships 0.44* Personal safety 0.43* Community connectedness 0.41* Future security 0.39* R 2 = 0.31 Adjusted R 2 = 0.27 *P < Unique variability = 0.083; Shared variability = people s satisfaction with various domains, which represent the first-level deconstruction of life as a whole. As emphasized in this paper, its advantages include easy and convenient use; a pre-testing protocol; simple and concretely worded questions; and the availability of reduced-choice response scales. It is important also to reiterate that the PWI s development is anchored in extensive use by a growing international community of scholars (International Wellbeing Group 2006), and that it evolved from an already widely-used life satisfaction scale, the ComQol, which received favourable appraisal. This examination of the PWI-ID s psychometric properties showed that it demonstrated adequate reliability and validity. This concurs with the psychometric results from the PWI in surveys of the Australian general population (Lau et al. 2005; Cummins et al. 2007). The scale had a Cronbach a-value of 0.76, and good test retest reliability. All of the scale items demonstrated an itemtotal correlation higher than the recommended minimum of 0.30, and one-factor emerged to explain 58% of the variance. The seven domains individually met the criterion for inclusion within the scale by contributing significant unique variance to the prediction of satisfaction with life as a whole. The PWI-ID scores obtained from the different response formats did not differ significantly, which indicates that when combined into one sample, they did not falsely over- or under-state the PWI score. Thus, it can be concluded that the PWI-ID is reliable and valid for individuals with mild and moderate levels of ID. Further, its psychometric profile identified previously amongst the general population has been confirmed in the present study. Interestingly, the sample s PWI-ID was 77.08, and the individual domains ranged from to These values fall within or slightly above the normative range for the PWI and domains respectively, and do not differ statistically to the results previously identified in the Australian general population. That the PWI-ID score was not compromised refutes the view that people with ID necessarily experience life satisfaction lower than the normative range. This finding supports previous research that also observed the tendency of individuals with and without a disability to report similar levels of life satisfaction. For example, Cummins (1995) noted that individuals with and without ID reported levels of satisfaction around 75 points on the scale. These data are consistent with the Theory of Subjective Wellbeing Homeostasis (Cummins 1998; Cummins et al. 2002; Cummins & Lau 2004). As explained earlier, this theory posits that humans have the cognitive ability to homeostatically maintain normal levels of SWB in diverse living conditions, unless there are adverse factors that are strong or persistent enough to cause homeostatic defeat. This proposition is based on extensive comparisons of SWB data across Western countries, which show a normative standard for population mean scores on life satisfaction, both for people with and without ID, that can be expressed approximately as 75 ± 2.5%SM (Cummins 1995, 1998; Cummins & Nistico 2002). Thus, two standard deviations on either side of the mean produces a normal range of points on the scale. The comparability of the PWI scores (i.e. SWB levels) in the present study with those from the general population can thus be interpreted to mean that the presence of an ID does not necessarily impinge on the SWB of the individual. The PWI-ID scores of the mild and moderate ID groups did not differ significantly, which shows that the degree of ID did not affect

9 284 Journal of Applied Research in Intellectual Disabilities SWB. In the context of the Wellbeing Homeostasis theory, this finding can be interpreted to mean that the increased level of ID in those individuals in the moderate range is not, of itself, sufficiently adverse in impact to cause homeostatic defeat. However, the comparison presented here is preliminary and exploratory, and serves to encourage further research into this topic. Indeed, the significant difference between the standard of living and health domains, which were higher in the group with mild ID, hints that the level of ID may play some role in mediating the components that comprise satisfaction with life as a whole. Although research is needed to delineate the psychological processes that mediate wellbeing homeostasis, at a teleological level it is linked to the need for a generally positive disposition that provides high levels of selfesteem and optimism. These, in turn, are presumably necessary to support an adequate level of motivation to achieve and maintain reasonable living conditions and perhaps to avoid the debilitating motivational consequences of depression. It is interesting to note that the lack of age and gender differences in the sample is also in agreement with previous findings from the general population (Cummins et al. 2007). There is also a prevailing notion amongst other researchers that these variables are not considered a very important influence on SWB (Diener et al. 1999). Despite these generally encouraging results, the business of creating a parallel version of the PWI that contained different wording for some items and different response scale format for some respondents, just has to be a fraught process. The best that can be hoped for is that the results are not so discrepant between these methodologies as to produce statistical differences from one another, and we may have achieved this. At a more subtle level, however, the two versions of the scale are actually different and so they cannot be making exactly the same measurement. To refine our understand of such differences requires rather exacting research which systematically varies the item wording and format, and records the measurement differences such changes incur. It seems to us this is a worthwhile endeavour since the PWI is such a robust scale with a solid theoretical basis and good empirical performance. The second major issue is our procedure of removing respondents who score at the top of the response scale over all of the seven domains. Such responding is best characterized as acquiescent responding (Sigelman et al. 1981a,b), which has been very well described over the years. While authors vary as to their estimates of the occurrence of this phenomenon, it is commonly reported to be present in some 20 30% of the responses from ID people, compared to some 3 5% from the general adult population. The PWI pre-testing procedures are, of course, intended to eliminate such respondents before they produce data for analysis. However, even despite these precautions, some get through the screening and this raises the question of what to do with their data. If they are included in the overall data set for analysis, they appear as extreme scores of 100 points on the PWI. Thus, even if they are valid data they will act to distort the psychometric description of the set being analysed. For this reason we removed them prior to analysis, as a conservative action to best preserve the character of the other data. However, there can be no absolute justification for this and each researcher is going to have to devise their own procedure. Despite these caveats, it is important to reiterate the significance of the present study. Existing scales (Heal & Chadsey-Rusch 1985; Schalock & Keith 1993; Test et al. 2005; Nota et al. 2006) cannot be norm-referenced back to the general population. The fact that the PWI can be used with the general population and also people with mild and moderate ID has far reaching implications. The SWB levels of different groups, and their determinants, can be compared and subject to application of the same theoretical principles. For example, this study showed that the SWB of people with ID was similar to the general population, and thus the normative scores of both samples can be explained by the same theoretical perspective (Wellbeing Homeostasis). Further use of the PWI-ID will aid the development of theoretical principles across sub-populations. In summary, the overarching aim of this paper was to introduce the PWI-ID as a reliable and valid tool for measuring the SWB of people with mild and moderate ID. Accordingly, the scale was exemplified and validated through its use in an Australian sample. The index demonstrated good reliability and validity. Further, a comparison of the results with previous research shows that the SWB level of people with ID approximates the normative range, and that there is no difference between people with mild and moderate ID. These results indicate that people who are intellectually disabled do not necessarily experience life quality lower than normal, which can be explained theoretically by the Theory of Subjective Wellbeing Homeostasis. The data provided by the PWI-ID for assessing wellbeing from the perspective of people with mild and moderate ID is fundamental to ensuring that their needs are met and to informing the planning and delivery of services.

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