Participants and procedures
A total of 559 people, who were recruited from a university and through a charity that offers support for eating disorders, participated in the EDE-QS validation part of the study first reported by Gideon et al. [4]. The same data are used for the secondary analysis reported in the current study. The majority of participants were women (80.86%) aged between 18 and 34 years (92.31%). Seventy-eight percent of the participants identified as White and 88.90% had tertiary (post-secondary school) education. Fifty-four (9.66%) participants self-reported currently having an ED diagnosis (i.e. they responded with Yes to the question “Do you currently suffer from an eating disorder (anorexia nervosa, bulimia nervosa, binge eating disorder, eating disorder not otherwise specified)?”). Of these, 7 (13%) met criteria for restrictive anorexia nervosa (AN-R); two (4%) for binge eating/purging AN subtype (AN-BP); eight (15%) for bulimia nervosa (BN); 10 (19%) for binge eating disorder (BED); and 27 (50%) for other specified feeding and eating disorders (OSFED) based on their responses on the EDE-Q and their Body Mass Index (BMI). Mean BMI in the total sample was 22.35 (SD=4.61), with 12% of participants classified as underweight, 72% as average-weight, 12% as overweight, and 3% as obese. Underweight individuals were not excluded from the subsample of participants who self-reported currently not having an ED diagnosis. Detailed information about recruitment processes and data collection have been presented elsewhere [4]. In brief, an invitation to participate was distributed to a large university in London, UK, and the study was advertised via the Beat - Beating Eating Disorders website, a UK’s eating disorder charity that offers support to people with current or former ED difficulties and their families. Participants provided informed consent and completed an online survey.
Measures
Eating Disorder Examination-Questionnaire Short. The EDE-QS was developed by Gideon et al. [4] as a 12-item version of the EDE-Q (see below) with a response scale ranging from 0 to 3, that captures essential symptoms of AN, BN and BED. The response scale was shortened during the development of the EDE-QS to reduce the cognitive demand and because respondents were not making full use of the 0-6 scale (some original categories were not used consistently with respondents’ ED severity), as observed in Rasch analysis results [see 4]. Scores of items are summed, ranging from 0 to 36 and higher scores indicate greater ED symptoms. ED symptoms are reported for the preceding seven days. Cronbach’s alpha obtained in this sample was .91 [4] indicating excellent internal consistency.
EDE-Q. The most recent version of the full EDE-Q [7] is a 28-item measure of ED symptoms and behaviors. Scores on each of four subscales (Restraint, Eating Concerns, Shape Concerns, Weight Concerns) and a Global score may be derived from items assessing core attitudinal features. Participants provide their answers on a scale from 0 to 6, with higher scores indicating greater frequency and/or severity of ED psychopathology over the previous 28 days. The EDE-Q has been validated in various clinical and non-clinical samples [8]. Previous studies showed a cut-off score for “probable” ED amongst young women of 2.3 (in conjunction with the occurrence of binge eating and/or excessive exercise) [9], and a clinical cut-off of a global EDE-Q score > 2.8 [6]. In the current study sample, Cronbach’s alpha was .96 for the global score and .84, .86, .93 and .88 for the Restraint, Eating Concern, Shape Concern, and Weight Concern subscales, respectively.
SCOFF. The SCOFF [10] is a 5-item measure used to screen for EDs in primary care. Items tap into key symptoms of AN and BN with a dichotomous (Yes/No) response scale. The number of “Yes” responses are summed to create a total score, with a score > 2 indicative of an ED. The SCOFF has been found to have good psychometric properties in international community samples [11]. Cronbach’s alpha in the current sample was .64 [4].
Clinical Impairment Assessment. The CIA (CIA 3.0) [12] was designed to measure psychosocial impairment associated with key ED features in the past 28 days. Sixteen items are answered on a 4-point Likert-type scale summed to compute the global score, with higher scores indicating greater perceived impairment. The CIA has previously been used for ED instrument validation [13,14] since clinical impairment has shown to be higher among ED clinical samples relative to healthy controls [15], thus supporting the instrument’s criterion validity [16]. The CIA has robust psychometric properties [12]. In the present sample, Cronbach’s alpha was .96.
Data analysis
The R package “epiR” was used to obtain sensitivity (the proportion of true cases correctly identified by the test), specificity (the proportion of true non-cases correctly identified by the test), positive predictive values (PPV; the proportion of individuals with positive test results who have an ED), and negative predictive values (NPV; the proportion of individuals with negative test results who do not have an ED). The PPVs and NPVs depend upon the prevalence of the disorder (e.g. if the prevalence is < 10% then PPV can be < 0.5) thus there is no defined criterion for classifying PPV or NPV as “acceptable,” or “good” [17]. The package “pROC” was used to compute the Area Under the Curve (AUC; the surface area under the curve which describes the relationship between sensitivity and specificity) statistic and confidence intervals (CI). AUC can obtain values from 0 to 1, with AUC of 0.50 classified as non‐informative; between 0.50 and 0.70 as less accurate; between 0.70 and 0.90 as moderately accurate; between 0.90 and 1 as highly accurate; and AUC = 1 is considered as perfect [18]. Two participants (0.36%) with missing values in the question about current ED diagnosis were excluded from the analyses (their Global EDE-Q scores were in a 30th and 38th percentile, indicating their removal would not markedly impact analyses as they did not have extreme results in this variable). Complete data were available for all other variables. Summation scores are more sensitive to missing data than other scoring methods [19], but as there were no missing data for the EDE-QS, summation was considered to be appropriate for this study. Global cut-off scores calculated as the average of item scores are also derived and presented in Table 1 in case of a necessary use by future researchers or clinicians, although for the simplicity in the following text we limit our discussion to global summation scores.
Using the self-reported ED diagnosis variable, the number of true positive, false positive, true negative, and false negative cases were determined separately for every possible cut-off score on the EDE-QS, and for the previously established range of cut-off scores on the EDE-Q (from 1.3 to 2.9) [20] and all possible SCOFF scores (from 0 to 5) [21]. Sensitivity and specificity rates were calculated for each of the possible cut-off score in these instruments, and PPV and NPV were also obtained. For the purpose of the current study, only findings for cut-off scores that yielded the highest levels of discriminatory parameters are reported, as the aim was to detect a cut-off score with high sensitivity, specificity and PPV. In addition to this, discriminatory parameters were also derived among female participants solely (n=452).