Unidimensionality and local independence
Results of the confirmatory models are presented in . These results suggest that fit indices from both the multidimensional model and the bifactor model were improved over the unidimensional model. Results from the three-factor model suggested substantial shared variability across the three factors, with an average correlation of .80 (SD = .06). Only the bifactor model produced fit indices that exceed suggested cutoffs across all three indices. When evaluating the need for a multidimensional model, we used the bifactor model to assess the relationship of each symptom with a primary dimension of PTSD after controlling for any relationships within each symptom domain. The symptoms’ relationship (e.g. factor loadings) to the primary dimension of PTSD across both models was quite similar, with the mean difference between loadings on a primary PTSD dimension across the two models being .014 (SD=.05). Thus, the bifactor model fits the data best, but controlling for criterion specific variability did not alter factor loadings on the primary dimension of PTSD substantially. Further, after controlling for loadings on the primary dimension, loadings within each criterion variable were reduced substantially relative to the multidimensional model. Our results suggest a unidimensional IRT model is unlikely to distort the item characteristics substantially and interpretation of item characteristics is unlikely to be biased due to these observed local dependencies. Although there was local dependence and a multidimensional model clearly fit the data better than a unidimensional model, after controlling for influences from multiple dimensions the items all clearly measured a strong common factor. Therefore, we selected an IRT model that captures variability along a single primary dimension of PTSD severity.
Factor loadings of different confirmatory factor models
Symptom-level estimates of problem severity
In , order of the severity parameter estimates mirror the raw frequencies of each symptom, but the scale better reflects the relative magnitude of differences between symptoms of the continuum. Severity estimates ranged from the lowest level of PTSD severity of −1.28 (symptom C1) to the highest level of severity at .855 (symptom C7). Several of the PTSD symptoms were associated with similar levels of PTSD severity, suggesting an overlapping or clustering of symptoms on the continuum of PTSD severity. For example, symptoms B2, C6, and C4 had similar severity ratings between −.232 and −.191. Despite some overlap, the PTSD symptoms indexed a broad range of PTSD severity. The discrimination index provides information about how well each item discriminates among individuals with PTSD symptoms. Discrimination of an item corresponds to the slope and thus how rapidly the probability of endorsing an item changes across levels of the underlying latent PTSD construct, with higher discrimination values reflective of steeper slopes. The majority of items discriminated similarly, with symptoms C3 (.848) and B1 (2.390) suggesting the least and most discriminating items, respectively.
Symptom parameter estimates and symptom bias for PTSD symptoms in ascending order of severity
shows item characteristic curves (ICC) to visually illustrate differences in the severity and discrimination characteristics among the DSM-IV PTSD symptoms. Examination of these curves provides a visual representation of the relationship between the increased likelihood of a symptom (e.g., the y-axis) and individual level of PTSD symptom severity (e.g., the x-axis). A discrimination within levels of PTSD severity is made when the symptom becomes more likely to be present than not (i.e., when the ICC crosses .50). For example, item C1 was more likely to be endorsed at lower levels of PTSD severity and item C7 was more likely to be endorsed at higher levels of PTSD severity. As illustrated in , there was a relatively large gap between the first 15 symptoms with the highest severity of −0.047 and the two most severe symptoms with severities of 0.836 and 0.855, suggesting that items in the current assessment have maximum reliability in capturing PTSD severity levels throughout the less severe end of the continuum.
Item characteristic curve (ICC) for PTSD symptoms.
Precision of the DSM-IV symptom index
Using the estimated measurement precision (i.e., information function from the model standard errors) of the DSM-IV PTSD symptoms, we can estimate the test information function (see ) to evaluate where on the continuum, the set of PTSD symptoms provide the most information or is most reliable in rank-ordering individuals. For reference, we calculated empirical Bayesian posteriori estimates for each level of PTSD among the 604 individuals with (PTSD +), and 585 individuals without a DSM-IV diagnosis (PTSD −). The average severity estimates for participants with PTSD was 0.55 (SD = 0.65) and −0.58 (SD = 0.79) for those without PTSD. Two vertical lines in position these group means with respect to the amount of information available from this set of DSM-IV symptoms. The 17 DSM-IV symptoms provide maximum information within a region of PTSD severity populated by those who do not meet the full DSM-IV diagnostic criteria. The amount of information decreases throughout the region populated by those with PTSD. Thus, the DSM-IV PTSD diagnostic classification rules place the diagnostic threshold beyond the reach of many symptoms. The lack of symptoms that characterize individuals above the diagnostic threshold may limit the ability to make further gradations within diagnosed groups. However, the PTSD symptoms do seem to capture variability within groups of individuals below a threshold for a diagnosis. For example, only 44 of the 585 participants reported no symptoms of PTSD and the median number of symptoms was 7.
Test information function for the 17 symptoms of Post Traumatic Stress Disorder. The vertical lines locate the average severity level for individuals with (PTSD +) and without (PTSD −) the DSM-IV diagnosis.
Gender influence on symptom endorsement
Men and women indicated similar patterns of symptom endorsements with no significant DIF on 10 of the 17 symptoms (p < .05), although on average women had slightly higher PTSD severity than men (Cohen’s d =.25). Women were more likely than men to report feeling distant emotionally (bmen=−0.14; bwomen =−0.45; DIF = .31, p<.05) and feeling easily startled (bmen= 0.18; bwomen = −0.04; DIF = 0.22, p<.05) at lower levels of PTSD. Conversely, men reported lack of plan for future (bmen= 0.80; bwomen = 1.20; DIF = −0.40, p<.05), unwanted memories (bmen=−0.79; bwomen =−0.56; DIF = −0.23, p<.05), unpleasant dreams (bmen=−0.22; bwomen = 0.00; DIF = −0.22, p<.05), and short-temper (bmen=−0.03; bwomen = 0.24; DIF = −0.23, p<.05) at lower levels of PTSD than women. Finally, reports of experiencing flashbacks did not discriminate across levels of PTSD as well among men when compared to women (amen= 1.15; awomen = 1.74; DIF = .59, p < .05).