OF AN ANNOYANCE SCALE FOR COMMUNITY NOISE ASSESSMENT

Transcription

1 Page number: 1 VALIDATION OF AN ANNOYANCE SCALE FOR COMMUNITY NOISE ASSESSMENT Ubaldo Ayr, Ettore Cirillo, Francesco Martellotta Politechnic of Bari, Applied Physics Institute Via Orabona, 4 I Bat-i, Italy address of lead author: u.ayr@cstar.poliba.it Abstract The validation of an annoyance scale for community noise assessment, proposed in a preceding paper, is described. This scale has to be used for a study about correlations between some noise indices and subjective responses of office workers. It is a seven points scale with descriptors. To carry out complete statistical analysis, correlating objective measurements and subjective responses, without biases it is necessary to use an interval scale, where distances between descriptors have meaning and are roughly the same. Such a scale can be obtained by using a unidimensional scaling technique, Thurstone method of equal appearing intervals, which is particularly suitable for this kind of analysis. In order to have a sample composition similar to that who the final questionnaire is addressed to, 64 subjects with medium-high educational background were asked to rate 18 descriptors chosen among Italian adjectives, including those used in the proposed scale. The application of Thurstone method led to the following results: the seven selected descriptors fall near integer values, approximate equal intervals and, finally, show seven clear peaks in the distribution of subjective responses, thus indicating fairly clear semantic distinctions. The initial hypotheses about descriptors have been in large part confirmed. INTRODUCTION When studying the performances of indices characterising subjective response to sensory stimuli, a fundamental role is played by the questionnaire for collecting individual ratings on analysed stimuli [ 1,2]. Questionnaires are made up by one or more scales on which the interviewees report their subjective sensation: typical scales are those for noise annoyance, for thermal and visual comfort, but similar scales are widely used in social sciences and psychology. The number of scale points should be sufficiently high in order to have a good distribution of the sample with respect to the examined phenomenon, and in particular if it is necessary to 2405

2 Page number: 2 pick up a group of dissatisfied well distinguished from the less annoyed part of the sample. However the number of points should not be greater than the maximum number of absolute distinctions that people are able to do about a given stimulus. Some psychometrical studies [3] maintain that man is unable to make more than seven absolute distinctions (that is without making comparisons between different unidimensional stimuli). In social sciences there is a widespread usage of ordinal scales, that is scales whose descriptors (associated to the scale points) can only be rank-ordered and for which the distance between the attributes has no meaning (in fact in this case we can only define the median and percentiles). In order to analyse mathematically the collected data it is necessary to associate numbers to descriptors. Besides, if parametric statistics are required it is important that the descriptors are chosen so that equal differences along the scale correspond to equal differences in perceived intensity of stimuli. Such a scale is called interval scale. An interval scale cannot be obtained simply assigning consecutive integers to the descriptors of an ordinal scale; in fact this is correct only if equal differences along the scale correspond to equal differences in perceived intensity of stimuli, but this is true only if descriptors have been chosen properly, using statistical procedures known as scaling techniques. On the contrary statistical analysis will be biased as far as the number of points on the scale grows. A possible alternative to this kind of scales are semantic differential scales in which the scale points are simply numbered with consecutive integers, while only the top and the bottom points are labelled, so leaving the subjects free to interpret the scale. In this case one has to explain clearly to the interviewed subjects that the central point on the scale represents the intermediate intensity between extreme values, and that points on the scale are equally spaced. The only drawback of this scale is that the lack of descriptors for some scale points could make the expression of judgements less easy when people are not accustomed in doing this kind of evaluations. If the required subjective judgement can be expressed by means of some degrees of opposite sensations, as it happens in thermal sensation scales, one can define a bipolar scale, symmetrically graduated around the central scale point, so halving the number of items to select. ANALYSIS OF SCALING TECHNIQUES Scaling is, according to S.S. Stevens definition the assignment of objects to numbers according to a rule. This rule is based on objects attitude towards a given concept. Objects can be nouns, adjectives or statements used to express the concept: in the rest of the paper they will be called descriptors or items. Three scaling methods are most frequently used in social sciences: Thurstone method of equal appearing interval, Likert method (or Summative scaling) and Guttman method (or Cumulative scaling). Among them only the first is suitable for validating interval scales, while the others are used for defining different kind of scales [4]. Thurstone developed also two other scaling techniques: the method ofsuccessive intervals and the method of paired comparisons [5]. The three methods differ in how the scale values for items are constructed (the last two are much more complicated), but in all three cases the resulting scale is rated the same way by respondents [4]. For these reasons the method of equal appearing interval is more conveniently used in the work presented here. First of all, it is necessary to define a set of candidate items (SO-100 preferably) (fig. la); they must be worded similarly and must not differ in grammar or structure (i.e. all statements 2406

3

4 Page number: 4 APPLICATION TO NOISE ANNOYANCE SCALE The aim of the study presented here is that of validating the choice of descriptors proposed in a preceding paper [ 11. The adopted scale is unipolar since noise has to be considered as a source of dissatisfaction more than satisfaction, therefore a bipolar scale could force the ratings toward only one direction of the scale. In the literature, scales with four [6], five [7], six [S], seven [9,10], ten and eleven points can be found. A study aiming to a standardised scale [ 1 l] reports that the optimal number of scale points should range between 7 and 11, since with less than 7 points a good distribution of the sample can hardly be obtained, while with more than 11 points subjects could be unable to make the necessary semantic and sensory distinction. Items adopted have been chosen starting from Levine study [ 131 who applied scaling techniques to an annoyance scale adopting English terms; these terms have been translated and adapted to Italian language giving the following descriptors (English translations are reported only as indication, but they cannot be adopted as items in a scale): irrilevante (irrelevant), Zeggero (light), moderato (moderate), intermedio (intermediate), considerevole (considerable), grande (big) and estremo (extreme), respectively associated to points from 1 to 7. This scale will be validated here, applying Thurstone method of equal appearing intervals. DESCRIPTION OF THE PROCEDURE In the study presented here, Thurstone method has been modified, adopting the mean and standard deviation respectively as measures of central tendency and of dispersion. These, compared to median and semi-interquartile difference, show a smoothed and continuous variation on changing data distribution and, in this sense, they result more stable [ 131. To validate the scale proposed in [ 11, other 11 items have been added to the previous 7, so obtaining 18 descriptors to submit to the subjects judgement. The new items are: scarso (scarce), minim0 (minimum), Zieve (faint), appena rilevante (hardly relevant), non molto grande (not very big), alquanto rizevante (rather relevant), rilevante (relevant), piuttosto grande (rather big), alto (high),forte (strong), enorme (enormous). It has not been possible to adopt, according to Thurstone criterion, a wider set of descriptors since it is not so easy to find in the Italian language such a great number of suitable terms. On the other hand, some adjectives could have been left out, but it must be noted that a greater complication could have made the questions less easy to comprehend. Once choices have been made, the questionnaire has been developed. It first asks general demographic data (age, sex, work, educational background) and then asks judges to express their rating on the noise annoyance degree expressed by each descriptor, assigning to each one a rating from 1 to 7, where 1 is the lowest degree of annoyance and 7 is the highest degree of annoyance. Descriptors have been presented in alphabetical order and ratings have been collected by means of a table with seven numbered columns; judges were asked to write each item in the column (pile) corresponding to the assigned rating. Since the capacity to operate semantic distinctions about descriptors depends on educational background of subjects [ 131, the sample composition has been chosen in order to match demographic composition and educational grade of the sample to which the questionnaire for noise annoyance assessment is addressed: that is university students and office workers 2408

5 ~ Ubaldo Ayr Page number: 5 (usually graduate or at least having got General Certificate of Education). Demographic composition of the sample is shown in fig. 2. Frequency distribution of each descriptor has been then determined. The next step has been that of choosing items that better approximate an interval scale. To reach this aim four criteria have been defined. One has to choose descriptors that: 1) have the mean falling near integ~r val.ues; 2) define equal Figure 2- Demographic composition of the sample amplitude Intervals; 3) have the lowest standard deviation; 4) have the mode equal to the assigned scale point and, above all, with a frequency clearly higher than other ratings received. The first two criteria ensure that the chosen descriptors are those the subjects think to better represent the scale points and, at the same time, ensure that intervals between consecutive descriptors are as uniform as possible. Third and fourth criterion ensure that there is a concentration of ratings around the mean vote and that this concentration is due to a clear prevalence of a single vote among the others. This means that there is a typical response, indicating a clear semantic distinction, in the sense that most of the interviewed subjects agree in assigning to a given descriptor a unique rating. If for a given scale point the four criteria are simultaneously satisfied by two or more descriptors, these descriptors have to be considered as synonyms and a fifth criterion is needed. One has to choose the item that judges rated more frequently at the given point on the scale. RESULTS Considering the whole data collected (tab. I and Fig. 3) it can be seen that the range of responses covers the seven point scale fairly evenly, also ifa concentration of items can be seen between points 1-2 and 5-6. Applying the aforesaid criteria the following descriptors have been selected. For scale point 1, "irrilevante" must be chosen, being its mean 1.03, its standard deviation 0.18 and its mode equal to 1. For scale point 2, "lieve" and "leggero" have the means respectively equal to 1.83 and Both items have the same distance from the integer value, a similar standard deviation (0.66 and 0.63 respectively), and the mode equal to 2 with a frequency of 40 and 38 respectively (higher than other ratings). "Leggero" is chosen because it assures a more uniform interval spacing, as it will be clear after the choice of point 3 of the scale. For scale point 3 "moderato", with a mean of 3.16, is the descriptor that falls nearer the integer value; its standard deviation is equal to 0.82 and its mode is

6

7 Page number: 7 For scale point 4 -the scale centre -"intermedio" has been selected, having the mean of 3.86, standard deviation of 0.47 and a good modal behaviour. For scale point 5, there are two descriptors, "rilevante" and "considerevole", which have a similar behaviour with respect to the first three criteria (tab. I), while the fourth implies the choice of"rilevante" due to its better modal behaviour. For scale point 6, "forte" is selected, it has the mean equal to 6.03, standard deviation equal to 0.59 and the mode equal to 6. Finally point 7 is associated with "estremo" which has a mean of 6.98 (63 subjects out of 64 rated it 7) and a standard deviation of Finally point 7 is associated with "estremo" Table 11- Noise annoyance scale which has a mean of 6.98 and a standard deviation of Mean S.D. Di{f (63 subjects out of64 rated it 7). Irrilevante It can be observed that the differences between Leggero the mean ratings of selected items are roughly equal to Moderato unity (tab. II). Non-uniform differences can be Intermedio observed between points 3-4 (0.70) and 4-5 (1.22). Rilevante Moreover, all selected items show a modal behaviour Forte indicating a clear semantic distinction, in the sense Estremo that most of the subjects agree in assigning to each item a single rating (fig. 4). One has to observe that if the median and semi-interquartile range had been used instead of the mean and standard deviation, according to Thurstone method, the choice of the best descriptors should have been more difficult ) :1 0- ~ 30 ll Irrilevante ~ Leggero ~ Moderato ~ Intermedio [j!] Rilevante FiI Forte [j!] Estremo Figure 4 -Best descriptors of noise annoyance. 2411

8 Page number: 8 CONCLUSIONS The validation of a subjective scale represents the first step in most of the studies on subjective comfort and in particular in studies whose aim is that of correlating subjective responses with objective parameters. In fact in order to obtain unbiased correlations an interval scale is needed; that is a scale with descriptors defining roughly equal intervals. The application of Thurstone method of equal appearing intervals led to the following results: the seven selected descriptors fall near integer values, approximate equal intervals and, finally, show seven clear peaks, thus indicating fairly clear semantic distinctions. In the end, it must be underlined that the proposed scale has been in large part confirmed, with two exceptions at scale points 5 and 6 where rilevante and tforte replaced considerevole and grande. ACKNOWLEDGMENTS This study has been carried on within the national interest program of scientific research Indoor Environment Engineering, partially granted by Italian Ministry of University and Scientific and Technological Research REFERENCES Ayr U., Cirillo E., Martellotta F., Sul rumore negli ambienti condizionati destinati a uffici, Proc. XXVII Convegno Nazionale AIA, Genova maggio 1999,285290; Ayr U., Cirillo E., Martellotta F., Indici di rumore e benessere acustico negli ambienti condizionati destinati ad uffici, Proc. Convegno AICARR, Padova 10 giugno 1999,99-108; Miller G.A., The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information, Psychological Review 63, 81-97, (1956), reprinted in Psychological Review, 101(2), (1994); Trochim, William M., The Research Methods Knowledge Base, 2nd Edition. Internet page, URL: kb/index.htm (version 9/4/1999); Thurstone L.L., A Law of Comparative Judgment, Psychological Review, 34, , (1927); reprinted in Psychological Review, 101(2), (1994). Keighley E.C., The determination of acceptability criteria for office noise, JSound Vib. 4(l), (1966); Tang S.K., Wong CT., Performance of noise indices in office environment dominated by noise from human speech, Appl. Acoust., 55(4), (1998); Beranek L.L., Criteria for office quieting based on questionnaire rating studies, J. Acoust. Sot. Am. 28(5), (1956); Griffiths I.D., Langdon F.J., Subjective response to road traffic noise, J. Sound Vib. 8 (l), 16 (1968); Hay B., Kemp K.F., Measurement of noise in air conditioned, landscaped offices, J. Sound Vib., 23(3), (1972); 11. Schultz T., Synthesis of social survey on noise annoyance, J. Acoust. Sot. Am. 64 (2), (1978); 12. Levine N., The Developement of an Annoyance Scale for Community Noise Assessment, J. Sound Vib. 74(2), (1981). 2412