There has long been a concern that considerable bias in survey research can stem from respondents providing answers that are partly determined by social influences, in particular social desirability
. While the influence of social desirability bias has been found to vary according to the survey method, telephone and personal interviews have been found to be particularly prone to socially desirable responding
[1–3]. Hence, social desirability bias may be a major threat to the validity of self-report outcomes data. Although there are several elements to its conceptualization, social desirability bias can generally be described as a response style exhibited by respondents who endorse items that represent traits and/or behaviors that they think stand for a socially acceptable or endorsed position
. Further, it can be differentiated between two dimensions: 1) the need for social approval, i.e. creation of a positive impression of oneself to receive approval from others (impression management), and 2) self-deception or defensiveness, i.e. avoidance of disapproval by denying socially undesirable traits and/or behaviors
[5–8]. Social desirability has been found to be related to demographic variables; it is more likely to be identified in older women
[9, 10], women of lower socio-economic status
[10, 11], and higher age
[12, 13]. Finally, social desirability has been found to be strongly related to the positive rating of the personal qualities of self, family and friends and not of ‘people in general’, the so-called ‘better than average’ effect
While social desirability bias has been a general concern in evaluations based on self-reports
, it may play a particularly important role in chronic disease health education interventions, in particular those that are offered to groups of people with chronic conditions who were initially unknown to each other. First, it is likely that individuals would be inclined to present themselves or certain health behaviors in a more positive light. This phenomenon would generally apply to any health-related outcomes assessment. Second, in the specific context of group-based interventions, it is intended that participants and course leaders build strong rapport during the intervention that may last several weeks or months. As a result, at the end of courses, participants may be inclined to provide socially desirable answers to endorse course leaders regardless of whether they truly benefited from the intervention. That is, participants may be aware that they are indirectly evaluating the performance of both the course leader and the organization and therefore provide socially desirable responses to appease leaders rather than showing how they really felt after graduating from the self-management course. Finally, in this setting, participants often fill out questionnaires in the presence of leaders and their peers which again may trigger socially desirable responses as they may feel pressurized to endorse the leaders’ performance. Hence, social desirability bias may have a particular influence on post-test scores and thus apparent change scores.
To measure the influence of potential socially desirable responses, several scales have been developed
[5, 15–18]. Of these, the Marlowe-Crowne (MC) Social Desirability scale
 is one of the most widely used indices
. It is commonly described as a measure of a person’s need for approval. Although the original authors defined the concept of social desirability in terms of two dimensions, i.e. need for approval and avoidance of disapproval
[6, 20], they conceptualized the MC scale as a measure of a single dimension
[6, 21]. However, subsequent studies found little support for this hypothesis, with results ranging from two-factor
[5, 22] to multi-factor solutions
[19, 21, 23–26]. While such findings cast some doubt on the measurement properties of the MC scale, these studies should be treated with caution. Only two studies applied rigorous psychometric statistical techniques to investigate the properties of the MC scale
[19, 21]. Moreover, the generalizability of studies is questionable as almost all samples consisted of students
[19, 24, 25, 27–29].
The original MC scale consists of 33 items. Therefore, for some respondents it may be a burden to complete, particularly if the scale is among a panel of scales. As a consequence, short forms have been developed, with Reynolds’ (1982) and Strahan and Gerbasi’s (1972) short forms being most frequently applied
[19, 21]. Commentaries on the usefulness of the short forms vary substantially. While some suggest that all are unsatisfactory
[19, 24], others show that they are improvements over the original
[25, 26, 28]. However, these studies should also be treated with caution. Apart from one study
 none applied rigorous statistical methods. Further, factor analyses on the short forms were generally aimed at confirming/rejecting the one-factor hypothesis, whereas none tested the scales for a potential two- or multi-factor solution. Of all short forms, Reynolds’ MC-C
 has been explored extensively
 and is one of the most frequently used short forms
[32–34]. It has generally been described as a reliable alternative to the full scale
[30, 31, 35] with acceptable internal consistency
[24, 25, 30, 31, 34].
In summary, social desirability bias has received frequent attention in the literature
[20, 36]. However, in view of its potential threat to the validity of scores derived from participants of health interventions, it is surprising that this bias has rarely been explored in contexts where social desirability is likely to be an important bias. Only two out of more than 100 controlled trials of chronic disease self-management courses considered social desirability as a potential covariate
. The aim of this study was to explore the influence of social desirability bias on change scores derived from data collected from groups of participants taking part in chronic disease self-management courses.