Interpreting Lucid ViSS results

Transcription

1 P a g e 1 Lucid Research Ltd 3 Spencer Street BEVERLEY East Yorkshire HU17 9EG United Kingdom Interpreting Lucid ViSS results T: +44 (0) F: +44 (0) Office@lucid-research.com Overview The ViSS reports module shows the extent to which the person s performance was disrupted by visually stressful stimuli. All scores are shown in seconds. The program calculates the difference in performance between the visually stressful and visually nonstressful conditions and carries out various statistical analyses on the data. A graphical scale of susceptibility to visual stress from low to extremely high makes it easy to interpret the results and a full statistical analysis is also given. An example reports screen is shown in Figure 4. The assessor or administrator may add comments, which will be included on the print-out of results thus providing a complete record of the screening on one convenient A4 page. For information on how to use the Reports module and to obtain print outs please refer to the Lucid ViSS Software Guide. A summary table for group reporting may also be obtained, which can be useful for spotting at a glance which children in a group are most at risk of visual stress. If desired the assessor or administrator may type in comments about the screening or other information regarding the case, together with recommendations or decisions for further action. This facility is accessed by a button on the Reports module screen and any comments added will be saved with the screening data and included on the print out of results (for further details of how to use this facility please refer to the Lucid ViSS Software Guide). Lucid ViSS Interpretation Guide November 2012

2 P a g e 2 Figure 4. Example ViSS main reports screen. Statistical analysis Outliers An outlier is defined as An unusual score in a distribution that is considered extreme and may warrant special consideration (Hinkle, Wiersma and Jurs, 2003, p. 737). In ViSS, outliers would be any exceptionally long or exceptionally short search times that could have an undue influence on the statistical calculation on which ViSS bases its decisions. Exceptionally long search times could be due to visual stress but they could be also due to loss of concentration or attention, or to distraction or boredom. Exceptionally short search times could be due to chance: i.e. their just happening to fixate on the target word at first glance, rather than having to search the array. It is important that when ViSS carries out its calculations as far as possible the difference in search times between the two conditions is due to the effects of visual stress and not to other factors. Hence before any statistical calculations are carried out ViSS first identifies and deals with any outliers. The method used in ViSS for identifying outliers is the one most commonly adopted in statistics, including statistical applications such as SPSS (see Hinkle, Wiersma and Jurs, 2003). This method establishes a reasonable lower boundary (RLB) and reasonable

3 P a g e 3 upper boundary (RUB) for the distribution of scores. The RLB is calculated as the 25 th centile score minus 1.5 times the interquartile range (i.e the difference between the 25 th and 75 th centile scores). The RUB is calculated as the 75 th centile score plus 1.5 times the interquartile range. Any score that falls below the RLB is treated as a fast outlier and replaced by the RLB score, and any score that falls above the RLB is treated as a slow outlier and replaced by the RUB score. Descriptive statistics ViSS calculates and displays the following descriptive statistics on the data set: Average search time for non-stressful items (plus standard deviation). 1 Average search time for stressful items (plus standard deviation). Percentage increase in search time from the non-stressful condition to the stressful condition (note that if this is negative it means that the child was faster, on average, in the stressful compared with the non-stressful condition). Number of incorrect non-stressful and stressful items (note that equivalent items will have been substituted by the program) Number of fast and slow outliers (note that these scores will have been replaced according to the procedure explained in Section 0). The Reasonable Upper and Lower Boundaries of the distribution (see Section 0). In addition, the raw data can be viewed. 1 The standard deviation is a conventional statistical measure of the variability of scores in a data set.

4 P a g e 4 Analytical statistics ViSS carries out and displays the following statistical analyses on the data: The t statistic, which reveals whether the difference between the non-stressful and stressful condition was statistically significant. The formula applied is based on the pooling variances method, which is appropriate when N<30 and when the two conditions are not independent (see Hinkle, Wiersma and Jurs, 2003). The probability level associated with the value of t (i.e. p <). By convention, the minimum level of probability is p<0.05, which means that the observed result would be likely to be obtained by chance fewer than five times in one hundred. When p<0.05 or less the result is said to be statistically significant. If p<0.01, this means that observed result would be likely to be obtained by chance fewer than once in one hundred occasions, and p<0.001 means that the observed result would be likely to be obtained by chance fewer than once in one thousand occasions. Obviously, the smaller the p value the greater confidence that can be placed in the result and the conclusions drawn from it. The effect size, which is an indicator of how important a statistical effect is, independent of sample size. The effect size measure used in ViSS is Cohen s d, which reflects the difference between the two means (visually non-stressful and visually stressful) as a function of the standard deviation of the sample (note that the pooled estimate of the standard deviation has been used in this case: see Olejnik and Algina, 2000, for discussion). An effect size of 1 means that the difference between the two means is equal to one standard deviation. Cohen (1988) advocates that effect sizes around d = 0.2 should be regarded as small, around d = 0.5 as medium and d = 0.8 or greater as large.

5 P a g e 5 Classification of risk How risk level is calculated The likelihood that the person suffers from visual stress or, to put it another way, the degree of susceptibility to the effects of visual stress is calculated by ViSS and displayed on a 7-point graphical scale with an associated verbal label. The classification algorithm that ViSS applies is based on the following data: the probability level associated with the t statistic percentage increase in search time from the non-stressful condition to the stressful condition the effect size. If the t statistic is not significant (i.e. p>0.05) or if the percentage increase in search time from the non-stressful condition to the stressful condition is less than 10%, ViSS will classify the child as having a low risk of visual stress. If, however, the t statistic is significant (i.e. p<0.05 or smaller) or if the percentage increase in search time from the non-stressful condition to the stressful condition is 10% or greater, ViSS will classify the person as exhibiting some risk of visual stress. The classification system for level of risk used by ViSS is shown in Table 2. The classification algorithm utilises information about the percentage increase in average search time from non-visually stressful to visually stressful items, the statistical probability level and the effect size. The scale ranges from 1 (low risk) to 7 (extremely high risk). The scale is differentiated by colour (red for the higher risks, etc.) to facilitate easy detection of children who are most in need of attention for visual stress. Classification label Table 2. Classification of risk of visual stress used in ViSS. Low Borderline Slight Moderate High Very high Extremely high Scale Colour Green Amber Amber Pink Red Red Red

6 P a g e 6 Interpreting the level of risk Where a person has a risk level in the Red categories ( High or greater risk) it is likely that they experience major disruption to reading and writing activities as a result of visual stress. It recommended that such individuals should be referred for optometric assessment with a view to prescription of coloured spectacles (see Section Error! Reference source not found.). However, if for any reason that is not possible or if there is some delay, coloured overlays should be provided (see Section Error! Reference source not found.) and, in addition, other remedial strategies, such as use of coloured paper (see Section Error! Reference source not found.), should be considered. Where a person has a risk level in the Pink category ( Moderate risk) it is likely that they experience a fair degree of disruption to reading and writing activities as a result of visual stress. It recommended that such people should be provided with coloured overlays (see Section Error! Reference source not found.) but that other remedial strategies such as use of coloured paper (see Section Error! Reference source not found.) should also be considered. Where a person has a risk level in the Amber category ( Slight or Borderline risk) it is possible that they experience some degree of disruption to reading and writing activities as a result of visual stress. These categories arise when the statistical results do not show a consistent pattern, but nevertheless there are some indications of susceptibility to visual stress. This could be because the difference between the two conditions (non-visually stressful and visually stressful) was marked but, overall, performance was extremely variable or irregular (resulting in high standard deviations and non-significant t statistic) Or it could arise when the effect size and/or the percentage increase in search time from the non-visually stressful to visually stressful was small but nevertheless the difference between the two conditions was very regular. Further investigation is recommended, especially inspection of the Individual Item Analysis (see Section 0). In such cases the Symptom Questionnaire given in the Appendix (see Chapter Error! Reference source not found.) can also be employed to assist the administrator in coming to a conclusion. Scores of 8 or more on this questionnaire are generally (but not invariably) associated with susceptibility to visual stress (see Singleton and Henderson, 2007b). However, caution should be exercised when questioning children (especially young children) about symptoms of visual stress as their answers can be misleading: see Section Error! Reference source not found.. If, after having investigated, the administrator is still uncertain regarding the appropriate conclusion, the rule of thumb is that where ViSS has classified a the person as

7 P a g e 7 having Slight risk then they should be treated as having some susceptibility to visual stress and provided with treatment (such as coloured overlays) for a trial period. But in cases where ViSS has classified the person as having Borderline risk, unless there is some confirmatory evidence of susceptibility to visual stress (e.g. from the Individual Item Analysis or from reported symptomatology) they should not be recommended for treatment, although it may be appropriate to tell parents that they might like to seek a second opinion by consulting an optometrist (see Section 0). Where a person has a risk level in the Green category ( Low risk) it is very unlikely that they experience disruption to reading and writing activities as a result of visual stress. Unless the person spontaneously reports major symptoms of visual stress, it is usually safe to assume that they do not require, and are unlikely to benefit from, use of coloured overlays or coloured spectacles.

8 P a g e 8 Individual item analysis The individual item analysis is a bar chart that shows a complete record of the child s performance on all 25 items of the test. This is accessed via a button on the main reports screen, and is also included on the one-page print out of results. Deviations from the overall average search time are displayed below the average line for fast times and above the average line for slow times (see Figure 5 for example). The individual item analysis enables the administrator to inspect the results more closely and examine the variability in the child s performance under the two conditions. This information can be especially helpful in cases where the person has been classified by ViSS as borderline or as having a slight risk of visual stress. Such cases will often arise where there is a fairly large difference in average performance between the two conditions but also a high variability in performance, resulting in a non-significant t statistic. Inspection of the individual item analysis should help resolve the matter as the presence of one or two extreme scores that may have unduly influenced the outcome should be apparent. Figure 5. Example ViSS Individual Item Analysis

9 P a g e 9 Interpreting the Individual Item Analysis The Individual Item Analysis enables the administrator to see at a glance how the person was performing across the whole test and to note differential response patterns in the two conditions. In cases where the classification of risk of visual stress made by ViSS is in the range Moderate to Extremely high, this information is useful to reassure the administrator that the classification is appropriate. When ViSS classifies a person as having a High or greater risk of visual stress the bars in the green sections of the chart (nonvisually stressful items) will mostly extend below the average line, and the bars in the yellow sections of the chart (visually stressful items) will mostly extend above the average line. This pattern may not be quite so obvious in cases of people classified as having Moderate risk of visual stress, but in this classification there will be some markedly longer search times among the visually stressful items. However, the main use of the Individual Item Analysis is to help clarify less certain cases, where ViSS has classified the degree of risk as Slight or Borderline. In most cases where people are classified as having Slight risk of visual stress there is likely to be greater degree of variation in search times, which can make the pattern in the Individual Item Analysis more difficult to discern. This can also happen in cases where children are classified as having Borderline risk of visual stress. However, the Borderline classification will often occur because of a modest (10 19%) increase in search times for the visually stressful condition that fails to reach statistical significance. Where the Individual Item Analysis suggests a build-up of visual stress (as indicated by an increasing trend to longer search times as the visually stressful condition continues) this should be taken as positive evidence of susceptibility to visual stress in Borderline cases (for example, see Case Study 5, Section Error! Reference source not found.). This pattern will also provide confirmatory evidence in cases classified as having Slight risk. Note that this build-up of visual stress during the test may even carry over to the final four non-visually stressful items so that these are longer instead being shorter as is more usually the case (e.g. Case Study 6, Section Error! Reference source not found.). However, administrators should beware that this pattern can also arise as a result of general fatigue. It such situations the safest course of action is usually to recommend a trial period of intervention, e.g. using coloured overlays with a subsequent followup, say in three months.

10 P a g e 10 Relationships between visual stress and other visual problems Many of the symptoms that characterise visual stress could also indicate the presence of other visual problems, including refractive and accommodative errors (e.g. short-sightedness, longsightedness, astigmatism, amblyopia) and errors of binocular coordination (e.g. strabismus) (see Evans, 2001, for review). Such problems are quite common in young children and may not have been detected. After children have been screened using ViSS and the results indicate a likelihood of visual stress, it is therefore important that the possibility of other visual problems is considered in addition to treatment for visual stress. Parents should be recommended to seek a full assessment for their child by a qualified optometrist. Previously known as ophthalmic opticians, optometrists are trained professionals who examine eyes, test sight, give advice on visual problems, and prescribe and dispense spectacles or contact lenses. They also recommend other treatments or visual aids where appropriate. Optometrists are trained to recognise eye diseases, referring such cases as necessary, and can also use or supply various eye drugs. All optometrists practising in the UK must be registered with the General Optical Council, the profession s regulatory body, and are listed in the Opticians Register. There are currently around 10,400 registered optometrists in the UK. To locate your nearest optometrist you can visit the website of the General Optical Council ( where there is a facility to search by postcode for the nearest optometrists to your location. When choosing an optometrist, the letters FCOptom or MCOptom after his or her name means that optometrist is a fellow or member of the College of Optometrists ( and must adhere to the standards of clinical practice set by that college. If it is decided that coloured spectacles are probably the best solution for the child (see Section Error! Reference source not found.), the optometrist may be able to prescribe these as well, but it is worth checking this before making an appointment. In some cases the child s General Practitioner may decide it is appropriate to refer the child to a hospital eye clinic for optometric assessment.