We also completed an analysis of differential item functioning (DIF, or “item bias”) by age, gender, and visual acuity. None of the Perceived Stress Scale items exhibited evidence of significant DIF (greater than 0.5 logits) by gender. Two items, “I felt nervous or stressed” (0.54 logits) and “I felt on top of things” (0.63 logits) showed some evidence of DIF by age. Younger participants found it easier to endorse the “I felt nervous or stressed” item and more difficult to endorse the “I felt on top of things” item than participants above the median age. Two items, “I was upset by things that normally do not upset me” (0.59 logits) and “I felt things were going my way” (0.64 logits) showed some evidence of DIF by better-eye visual acuity. Participants with visual acuity worse than the median value found it easier to endorse the “I was upset by things that normally do not upset me” item and also the “I felt things were going my way” item. There was no significant correlation between better-eye visual acuity and Perceived Stress Scale person measure (r = −0.04, P = .06).
A nine-item form of the Perceived Stress Scale demonstrated acceptable psychometric properties in patients being managed for age-related macular degeneration (AMD). Over the past 30 years, various versions of the Perceived Stress Scale have been used in a wide range of populations. This is the first evaluation of the Perceived Stress Scale in people with AMD of which we are aware. The Rasch analysis presented here showed that the questionnaire demonstrated reasonable measurement precision, evidence of unidimensionality, and a properly functioning response category structure. The removal of one item from the ten-item version slightly improved measurement precision and unidimensionality in this analysis. However, there may be cases in which leaving a slightly misfitting item in the set is not harmful to measurement and may be desirable. In future studies, investigators using the Perceived Stress Scale should consider the fit of each item and make decisions regarding item inclusion or exclusion based on the results of their own analyses, taking into account the various indicators of scale validity. These findings provide support for the Perceived Stress Scale’s potential usefulness for patients with AMD.
A psychometrically sound, easy-to-administer questionnaire such as the Perceived Stress Scale is important for use with patients with AMD, given the evidence for increased rates of psychological symptoms in the population. Scores on the Perceived Stress Scale have been shown to be related to increases in cortisol levels, susceptibility to infection, shortened telomeres, increased pro-inflammatory cytokines, slow wound healing, and other negative health outcomes.42–46 Additionally, there are a variety of stress reduction treatment protocols that have been shown effective in reducing stress and improving health outcomes.47–51 Though depression has been examined in people with AMD in several studies, there has been relatively little exploration of perceived stress. There have, however, been some studies that used scales with related content. One scale that measures some similar constructs and has been used in AMD is the Profile of Mood States (POMS).52 The POMS scale asks subjects to rate how they feel at the time of survey for a number of words that describe feelings, including “tense,” “angry,” and “confident.” The POMS has been used in studies of interventions for patients with AMD.53 It differs from the Perceived Stress Scale in several ways, including that it is meant to target seven different constructs whereas the Perceived Stress Scale is meant only to be a measure of perceived stress, it is considerably longer, and its response scale is different and covers a different time period (at the time of survey for the POMS vs. in the last week for the Perceived Stress Scale). The POMS is a proprietary instrument, whereas the Perceived Stress Scale is available for use at no charge. The Impact of Vision Impairment instrument contains a socioemotional subscale with items dealing with worry about worsening eyesight and concern about coping with everyday life, but additionally others that target content similar to depression scales (sad or low, felt like a burden, lonely or isolated).
We found that the Perceived Stress Scale targeted people with somewhat higher perceived stress levels than those of study participants. Previous work54 on trends in Perceived Stress Scale scores by demographics provide some insight into why this might have occurred. First, it has been reported that Perceived Stress Scale scores generally decline with increasing age, and the mean age in our sample was 82 years. Stress levels have also been shown to be lower in Whites, who are at higher risk for AMD and comprised the vast majority of study subjects. Additionally, Perceived Stress Scale scores would be expected to decline with increasing levels of education, and this study sample likely had a higher average education level than the general population. Approximately 42% of the sample had at least a bachelor’s degree, compared with about 27% of people 65 years or older in the United States, as reported in the 2015 U.S. Population Survey.55 Further work should be done to determine how the targeting of the Perceived Stress Scale might be different in other samples of people with AMD.
An analysis of differential item functioning (DIF), or item bias, detected two Perceived Stress Scale items that had slightly more than desirable DIF by age, and two with slightly higher than desirable DIF by better-eye visual acuity. Younger participants found it easier to endorse the “I felt nervous or stressed” item and more difficult to endorse the “I felt on top of things” item than participants above the median age, whereas participants with visual acuity below the median level found it easier to endorse the “upset by things that normally do not upset me” item but also the “felt things were going my way” item. The presence of DIF can negatively affect measurement, and our findings indicate that it may be desirable to make accommodations in scoring procedures or discard the items in future studies using the Perceived Stress Scale.56
One other recently published study by Nielsen et al. evaluated a Danish version Perceived Stress Scale using a different form of the Rasch model than the one used in this study.22 This study was performed with a population-based sample in Denmark, and it used the partial credit Rasch model, which allows for different category thresholds for each item. It found that there was evidence of multidimensionality related to differences between positively and negatively worded items, and that there were problems with overall model fit (as assessed with a global chi-square test) and category structure for some items. Using the rating scale Rasch model,40 we found no disorder in the response category thresholds. We found that the eigenvalue of the first contrast of a principal component analysis of model residuals for the nine-item set was 1.9, just below the 2.0 threshold which has been recommended as an indicator of acceptable unidimensionality. However, inspection of the factor loadings on this first contrast showed that the positively worded items all loaded together, a finding that is generally consistent with Nielsen et al. and others9,57 that there may be fundamental differences in responses to the positively and negatively worded items. In fact, Nielsen et al. suggested developing and testing a version of the Perceived Stress Scale in which all of the items are negatively worded, which may be a strategy for dealing with this issue.
A limitation of the study is that some participants were administered the Perceived Stress Scale by study staff if they indicated they could not comfortably complete it alone secondary to difficulty reading the questions. Survey questions were printed one per page in 36-point font to avoid this problem as much as possible, but the nature of AMD’s effect on reading makes administering surveys in print difficult. It is possible that some of these participants answered some questions in a different way than they would have had they been able to complete the survey alone. Strengths of this study include that it is the first study of which we are aware that has assessed the Perceived Stress Scale, an instrument that is in very common use throughout many health-related fields and has been shown to be related to a number of important health outcomes in people with AMD. The study is also only the second of which we are aware to analyze the Perceived Stress Scale using a Rasch analysis, the benefits of which have been demonstrated frequently.
Bradley E. Dougherty
338 W. 10th Ave
Supported by National Eye Institute/National Institutes of Health, Department of Health and Human Services grant K23EY022940.
Received May 16, 2016; accepted November 15, 2016.
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