Psychological Trauma

Journal of Traumatic Stress, Vol. 13, No. 2, 2000

Comparison of the PTSD Symptom Scale-Interview Version and the Clinician- Administered PTSD Scale

Edna B. and David F. Tolin’

The Clinician-Administered PTSD Scale (CAPS) is one of the most frequently used measures of posttraumatic stress disorder (PTSD). It has been shown to be a reliable and valid measure, although its psychometric properties in nonveteran populations are not well known. One problem with the CAPS is its long assess- ment time. The PTSD Symptom Scale-Interview Version (PSS-I) is an alternative measure of PTSD severity, requiring less assessment time than the CAPS. Pre- liminary studies indicate that the PSS-I is reliable and valid in civilian trauma survivors. In the present study we compared the psychometric properties of the CAPS and the PSS-I in a sample of 64 civilian trauma survivors with and without PTSD. Participants were administered the CAPS, the PSS-I, and the Structured Clinical Interview f o r DSM-IV (SCID) by separate interviewers, and their re- sponses were videotaped and rated by independent clinicians. Results indicated that the CAPS and the PSS-I showed high internal consistency, with no direr- ences between the two measures. Interrater reliability was also high f o r both measures, with the PSS-I yielding a slightly higher coeficient. The CAPS and the PSS-I correlated strongly with each other and with the SCID. Although the CAPS had slightly higher specijcity and the PSS-I had slightly higher sensitivity to PTSD, overall the CAPS and the PSS-I peqormed about equally well. These results suggest that the PSS-I can be used instead of the CAPS in the assess- ment of PTSD, thus decreasing assessment time without sacrijcing reliability or validity. KEY WORDS: posttraumatic stress disorder; CAPS; PSS-I; SCID.

I Center for Treatment and Study of Anxiety, Department of Psychiatry, University of Pennsylvania, 3535 Market Street, 6th Floor, Philadelphia, Pennsylvania 19104.

*To whom correspondence should be addressed.

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0894-98h7/00/0400-0181$18.00/1 c 2000 International Soclefy for Traumatic Streaa Sludie\

182 Foa and Tolin

One of the most widely used measures of posttraumatic stress disorder (PTSD) is the Clinician-Administered PTSD Scale (CAPS; Blake et al., 1990), often re- ferred to as the “gold-standard” measure for PTSD. The CAPS is a semistructured interview that measures the 17 symptoms of PTSD. Each symptom is assessed using two questions (for a total of 34 items): one measuring frequency of the symptom’s occurrence, and the other, its intensity (e.g., distress or functional im- pairment). To ascertain validity of response, each question is followed by a number of probe questions that aim at clarifying the frequency and intensity of the symp- tom. CAPS responses are used not only for making a dichotomous PTSD diagnosis, but also for quantifying the seventy of PTSD. The CAPS was originally devel- oped for use with combat veterans and most studies of its psychometric properties have used this population (e.g.. Blake et al., 1990). More recently, to our knowl- edge only one study (Blanchard et al., 1995) has examined the reliability of the CAPS in civilian populations, yielding high to very high reliability coefficients. Hovens et al. (1994) found high reliability and moderate validity coefficients us- ing a Dutch-language version of the CAPS. However, that sample contained both civilians and combat veterans; therefore, it is difficult to determine whether the same results would apply to a civilian sample.

Although the CAPS has excellent psychometric properties, as noted by Newman, Kaloupek, and Keane (1996), its major drawback is the substantial amount of time required for its administration due to its large number of items. Depending on the interviewee’s symptom picture, administration of the CAPS can take 40 to 60 min.

One potential alternative to the CAPS is the PTSD Symptom Scale-Interview Version (PSS-I; Foa, Riggs, Dancu, & Rothbaum, 1993). The PSS-I is a semistruc- tured interview that consists of 17 items, corresponding to the 17 symptoms of PTSD. Unlike the CAPS, frequency and intensity of symptoms are combined on the PSS-I into a single rater estimate of seventy. The reason for combining these two dimensions is that some symptoms lend themselves more readily to frequency estimates (e.g., nightmares) whereas others are more readily described in terms of intensity (e.g., hypervigilance). Excellent reliability and validity have been found for the PSS-I using female victims of rape and nonsexual assault (Foa et al., 1993). Because the PSS-I consists of only 17 items (compared to the CAPS’S 34), its administration time is relatively short, approximately 20 to 30 min.

The purpose of the present study was to compare the psychometric proper- ties of the CAPS and the PSS-I using a sample of individuals with and without PTSD who had experienced a variety of traumatic events. We administered the two interviews and compared the resulting diagnostic status and symptom severity to one another and to that yielded by the Structured Clinical Interview for DSM-IV (SCID; First, Spitzer, Gibbon, & Williams, 1995). If the CAPS and the PSS-I show similar reliability and validity to each other, then the PSS-I may be a useful alternative to the CAPS when resources are limited.

PSS-I versus CAPS 183

Method

Participants

Participants were a convenience sample of 12 clinic patients and 52 non- clinical adult volunteers (total = a), recruited from a relatively heterogeneous community sample in the greater Philadelphia area. The clinic patients were re- ceiving outpatient treatment for PTSD; the remainder responded to advertisements and requests for volunteers at community presentations. All participants were re- imbursed $30 for their participation.

Fifty-three percent of the participants were female, and 47% were male. Mean age was 37 years (SD = 10). Fifty-two percent were Caucasian, 39% were African American, 3% were Hispanic, 5% were Asian American, and 1% were other ethnicity.

All participants reported experiencing a traumatic incident that met Crite- rion A of the DSM-ZV (American Psychiatric Association, 1994) PTSD diagnosis. The sample included a heterogeneous range of traumatic experiences, with per- centages as follows: rape 18%, other sexual assault 8%, nonsexual assault 32%, fire/explosion 11 %, accident 14%, and other trauma 17%. None of the participants were combat veterans.

Measures

PSS-I (Foa et al., 1993). The PSS-I is a semistructured interview designed to assess current symptoms of PTSD as defined by DSM-ZV (American Psychi- atric Association, 1994) criteria. The PSS-I consists of 17 items corresponding to the 17 symptoms of PTSD, and yields a total PTSD severity score as well as reexperiencing, avoidance, and arousal subscores. Each item consists of one brief question. The participant’s answer is rated by the interviewer from 0 (Not at all) to 3 (5 o r more times p e r week/Very much). Total severity scores on the PSS-I are based on sums of the raw items. Symptoms measured by the PSS-I are considered present if they are rated as 1 (Once p e r week or less/A little) or greater.

Factor analysis of the PSS-I yielded three factors: avoidancehrousal, numb- ing, and intrusion (Foa, Riggs, & Gershuny, 1995). Internal consistency coefficients for the PSS-I subscales range from .65 to .71 in a sample of female sexual and nonsexual assault victims. Test-retest reliabilities range from .66 to .77 over a 1-month period. Interrater reliabilities range from .93 to .95. The PSS-I shows good concurrent validity, as indicated by significant correlations with measures of PTSD symptoms, depression, and general anxiety (Foa et al., 1993).

CAPS (Blake et al., 1990). The CAPS is a semistructured interview designed to measure symptoms of PTSD according to DSM-ZZZ-R (American Psychiatric

184 Foa and Tolin

Association, 1987) criteria. The CAPS has 34 symptom-oriented items, each rated on a 5-point scale, which correspond to the 17 symptoms of PTSD. The CAPS yields two total scores, one for frequency and one for intensity, as well as two sub- scores for each of the reexperiencing, avoidance, and arousal subscales. The anchor points of the scales vary according to symptom, but higher numbers consistently indicate either higher frequency or intensity of the symptom.

In addition to having separate ratings of frequency and intensity, the CAPS differs from the PSS-I in that it includes questions to be used as prompts if the assessor needs further clarification. The CAPS also can be used to assess both lifetime and current PTSD symptomatology; however, for the purposes of the present study only current symptoms were assessed.

Previous research indicates that the CAPS shows excellent interrater reliabil- ity ( r = .92 to .99) for all three subscales in combat veterans. Internal consistency coefficients range from .73 to 3 5 . The CAPS shows good concurrent validity, as indicated by significant correlations with self-report measures of PTSD symptoms (Blake et al., 1990). Thus, the CAPS appears to be a reliable and valid mea- sure. Partly because of the complexity inherent in obtaining separate scores for frequency and intensity, several scoring rules have been proposed for the CAPS (Blanchard et al., 1995; Weathers, Ruscio, & Keane, 1999). With motor vehicle accident victims, Blanchard et al. (1 995) used three scoring rules: a liberal rule requiring a score of at least 2 as the sum of the frequency and intensity ratings for a given item; a moderate rule requiring a score of 3, and a conservative rule requiring a score of 4. As expected, rates of PTSD were highest using the liberal rule, and lowest using the conservative rule.

With combat veterans, Weathers et al. (1999) examined nine different ra- tionally and empirically derived scoring rules for the CAPS. Three scoring rules were particularly recommended: the “F 1/12’’ rule (liberal rule) required a frequency score of at least 1 and an intensity score of at least 2 for each item. This rule was recommended for screening purposes to avoid false negatives. When false positives and false negatives are equally undesirable (e.g., differential diagnoses), the “SCID Symptom-Calibrated (SXCAL)” rule was recommended. The SXCAL rule uses the optimally efficient severity-score cutoff for each item for predicting the pres- ence or absence of the corresponding PTSD symptom on the SCID (Weathers et al., 1999). When false positives needs to be minimized (e.g., confirming a diagnosis), the conservative “Clinician-Rated 60” scoring was recommended. Accordingly, a symptom is considered present if the combination of frequency and intensity for that item was rated as present by at least 60% of a sample of 25 expert clinicians (Weathers et al., 1999). This resulted in different cutoff scores for each CAPS item.

SCID. (First et al., 1995). The SCID is a structured interview measuring DSM-ZV (American Psychiatric Association, 1994) symptoms of PTSD. The SCID diagnosis of PTSD showed acceptable agreement with indexes obtained from previously validated assessment instruments included in the National Vietnam Veterans Readjustment Study (Kulka, Schlenger, Jordan, & Hough, 1988), and

PSS-I versus CAPS 185

was identified previously as an instrument of choice in the assessment of rape- related PTSD (Resnick, Kilpatrick, & Lipovsky, 1991).

On the SCID, each symptom is assessed using one question, and the inter- viewer rates each symptom on a 3-point scale: absent or false, subthreshold, and threshold or true. Symptoms are considered present if they are assigned the latter rating.

Procedure

Thirty-nine participants were interviewed by two clinicians. The first inter- viewer queried the participant about trauma history and assisted the participant in identifying a single traumatic even that would be the focus of the interview. Participants reporting more than one traumatic event were instructed to select the most bothersome incident for this interview. Participants were also instructed to refer to the same traumatic event for all interviews, and reviews of videotapes indi- cated that all participants complied with this instruction. One interviewer used the CAPS and the other, the PSS-I. The order of administering the two instruments as well as which instrument would be used by which clinician were each determined randomly. Over the course of the study, 22 clinicians conducted the interviews. Participants were instructed to refer to the same traumatic event in both interviews. Clinicians were instructed not to discuss a participant’s interview with one another until all interview data had been collected for that individual.

All interviews were videotaped. The videotapes were reviewed by at least two raters who did not have access to the interviewers’ ratings. These raters scored the CAPS and the PSS-I on the basis of the participant’s responses in the videotapes; later, these ratings were compared to those of the interviewer.

To assess convergent validity with the SCID, an additional 25 participants were administered the CAPS and the PSS-I as described above as well as the PTSD module of the SCID; the latter was administered by a third clinician. The order of the three interviews and the assignment of the clinician-interviewer were determined randomly.

All interviewers and raters were doctoral or master’s level clinicians who were trained in the use of both instruments by the instruments’ developers (Dr. Edna Foa for the PSS-I and Dr. Frank Weathers for the CAPS). To ensure standard admin- istration and scoring, interviewers and raters met weekly to review the interviews, ascertain adherence to interview procedures, and resolve scoring discrepancies.

Results

Kolmogoroff-Smirnov tests of the distribution of scores on the PSS-I and CAPS indicated that scores were not normally distributed. Therefore, nonpara- metric statistics were used wherever possible.

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Table 1. Cronbach’s Aluha Coefficients for the PSS-I and the CAPSa

Foa and Tolin

PSS-I CAPS

No. ofItems (Y No.ofItems (Y

Total score 17 3 6 34 .88 Reexperiencing subscale 5 .70 10 .70 Avoidance subscale 7 .I4 14 .76 Arousal subscale 5 .65 10 .7 I

“PSS-I = PTSO Symptom Scale-Interview Version; CAPS = Clinician- Administered PTSD Scale.

Reliability of the PSS-I and the CAPS

Internal consisrency. Cronbach’s alpha was calculated on PSS-I and CAPS total scores and subscale scores. Because the CAPS includes two items per symp- tom (frequency and intensity) and the PSS-I includes only one item, we used a dichotomous coding of each item to indicate its presence or absence. By doing so, we controlled for the different number of items.

Alpha coefficients for the PSS-I and the CAPS are shown in Table 1 . Internal consistency was good to very good for all scales and subscales of both the PSS-I and the CAPS, with the alpha coefficient ranging from .70 to .88 for the CAPS and from .65 to .86 for the PSS-I. Thus, the internal consistency of the PSS-I and the CAPS were comparable.

To further examine internal consistency, we correlated each item’s raw score with the total score. The average item-total correlation for the PSS-I was S 9 , with correlations ranging from .11 to .74. For the CAPS, the average item-total correlation was .52 with arange of .21 to .68. On both interviews, the item reflecting the symptom of “inability to recall an important aspect of the trauma” showed low correlations with the total score (on the PSS-I, p(63) = . l I , p = .39; on the CAPS, p(63) = .21, p = .09). Thus, on this index of internal consistency, the CAPS and the PSS-I were again quite similar.

The correlations among the three symptom cluster and the total severity scores for the CAPS and the PSS-I are presented in Table 2. The intercorrelations among subscales for each instrument were moderate to high and the overall picture was again quite similar.

Interviewer-rater reliability. Interviewer-rater reliability was calculated by comparing the interviewer’s ratings to those of the videotape raters. Because there were several raters and one interviewer for each instrument, reliability coefficients were calculated as follows: First, each videotape rater was assigned a number (1-4). Next, Spearman correlation coefficients were calculated between the interviewer and rater 1, the interviewer and rater 2, and so on. The resulting coefficients were translated into Fisher’s z scores (Rosenthal & Rosnow, 1984) and averaged. Then, the average z score was translated back to p to yield a single interrater reliability

PSS-I versus CAPS 187

Table 2. Spearman Correlations Among the Subscales of the PSS-I and the CAPS

Subscale Total Score Reexperiencing Avoidance

PSS-I Reexperiencing .82* Avoidance .92* .63* Arousal .88* .63* .71*

Reexperiencing .87* Avoidance .90* .68* Arousal .88* .67* .70*

CAPS

* p < ,001.

Table 3. Interviewer-Rater Reliability Coefficients and Percentage Agreement for the PSS-I and the CAPS

~~ ~ ~

Pss-I CAPS p % Agreement p %Agreement

Reexperiencing subscale .93* 99.2 .89* 92.5 Avoidance subscale .91* 97.5 .86* 88.5 Arousal subscale .92* 94.2 .8 1 * 93.4 Total score/PTSD diagnosis .93* 98.3 .95* 86.6

coefficient. Percentage of rater agreement for the presence or absence of each symptom was calculated by averaging the agreement of each videotape rater with that of the interviewer. Rater agreement for the CAPS was calculated using the F1/I2 rule (Weathers et al., 1999), since this was the original scoring rule reported by Blake et al. (1990). Using other scoring rules for the CAPS did not change interrater reliability significantly.

Table 3 presents the reliability coefficients of the total scores and for each subscale, as well as the percentage of rater agreement on the presence or absence of each symptom cluster and PTSD diagnosis. As can be seen in Table 3, both the CAPS and the PSS-I showed excellent interviewer-rater reliability. There were no substantial differences between the two measures, although the PSS-I showed consistently higher rates of agreement between raters for both the correlations and percentage agreements.

Validity of the PSS-I and the CAPS

Frequency of PTSD diagnosis. Thirty (46%) of participants met diagnostic criteria for PTSD according to the PSS-I. Rates of PTSD with the CAPS varied

188 Foa and Tolin

Table 4. Diagnostic Agreement Between the CAPS and the PSS-I

PSS-SR

CAPS Scoring Rule % Agreement Kappa

Liberal (Weathers) 83 .65 Moderate (Weathers) 78 .55 Conservative (Weathers) 70 .38 Liberal (Blanchard) 86 .I2 Moderate (Blanchard) 84 .68 Conservative (Blanchard) 80 .58

Note. Blanchard = Blanchard et al. (1995); Weathers = Weathers et al. (1999).

Table 5. Correlations Between the Subscales of the CAPS and the PSS-I

CAPS

Reexperiencing Avoidance Arousal PSS-I Total Score Subscale Subscale Subscale

Total score .87* .76* .74* .76*

Avoidance subscale .75* .55* .75* .64* Arousal subscale .17* .64* .63* .78*

Reexperiencing subscale .76* .79* .57* .64*

Note. Correlation coefficients between scales measuring the same symptoms on both interviews are italicized. * p < .001.

according to the scoring rule used. Using the Blanchard et al. (1995) diagnostic rules, 33 (5 1 %) were diagnosed with PTSD with the liberal rule, 2 8 (43%) with the moderate rule, and 2 1 (32%) with the conservative rule. Rates of PTSD diagnosis on the CAPS also vaned across the different scoring rules described by Weathers et al. (1999). Using the liberal rule, 23 (35%) were diagnosed with PTSD; 20 (31%) with the moderate rule, and 11 (17%) with the conservative rule. Thus, PTSD rates yielded by the PSS-I were similar to those obtained with the Blanchard et al. moderate scoring rule. Both the Blanchard et al. and the PSS-I rates were somewhat higher than those emerging from the Weathers et al. rules.

Concurrent vuiidity. A high correlation of p = .87 (p < .001) was found be- tween the CAPS and the PSS-I for the total score. Agreement across the two measures on PTSD diagnosis varied according to the CAPS scoring rule used (see Table 4). Table 5 displays the Spearman correlations between the interview scales.

Convergent validity. To assess convergent validity, CAPS and PSS-I scores were compared to the PTSD section of the SCID. Spearman correlation coefficients indicated that the SCID total score correlated strongly with the CAPS total score p ( 2 3 ) = 3 3 , p < .001; and PSS-I total score, p ( 2 3 ) = .73, p < .001. To examine whether the correlation between SCID and CAPS total scores was greater than the correlation between SCID and PSS-I total scores, a Hotelling’s t test was performed. Results were not significant: t ( 2 4 ) = 1.68, p > .05.

PSS-I versus CAPS 189

Table 6. Agreement Between the SCID and the CAPS and the PSS-I

CAPS

Liberal Moderate Conservative PSS-I Scoring Rule Scoring Rule Scoring Rule

Standard SCID Subscale Blanchard Weathers Blanchard Weathers Blanchard Weathers Scoring Rule

Total Score %Agreement 80 84 80 88 88 84 80 Sensitivity 0.86 0.71 0.71 0.71 0.71 0.43 0.86 Specificity 0.78 0.89 0.83 0.94 0.94 1 .oo 0.78 Kappa .56 .60 .52 .69 .69 .5 1 .56

%Agreement 84 80 84 84 80 56 92 , Sensitivity 0.85 0.80 0.85 0.85 0.80 0.45 0.90

K"PP" .57 .49 .57 .57 .49 2.5 .78

Reexperiencing

Specificity 0.80 0.80 0.80 0.80 0.80 1.00 1 .oo

% Agreement 80 84 80 84 88 88 80 Sensitivity 0.88 0.75 0.75 0.62 0.75 0.62 0.88

Avoidance

Specificity 0.76 0.88 0.82 0.94 0.94 1.00 0.76 Kappa .58 .63 .56 .61 .7 1 .69 .58

%Agreement 64 84 68 72 80 72 76 Sensitivity 1 .oo 1 .oo 1.00 1.00 1.00 0.50 I .oo Specificity 0.31 0.69 0.39 0.46 0.62 0.92 0.54 Kappa .30 .68 .38 .45 .6 1 .43 .53

Arousal

Notes. Blanchard = scoring rule from Blanchard et al. (1995); Weathers = scoring rule from Weathers et al. (1999). Percent agreements are calculated to reflect whether participants met or exceeded the symptom count for the DSM-IV diagnosis.

When data were analyzed according to the presence or absence of symptoms rather than a continuous score, the results varied according to the scoring rule used. As shown in Table 6, both the PSS-I and the CAPS showed moderate to strong agreement with the SCID. The PSS-I showed somewhat higher sensitivity, whereas the CAPS showed somewhat higher specificity, especially using more conservative scoring rules. On both the CAPS and the PSS-I, the arousal subscales showed high sensitivity but relatively low specificity with the SCID. Given the strong agreement between the PSS-I and CAPS on the arousal subscale (r = .78), the low specificity may reflect a psychometric weakness of the SCID rather than of the two instruments in question. Overall, however, the CAPS and the PSS-I performed quite similarly in relation to the SCID.

Interview duration. Precise interview times were available for 42 sets of in- terviews. Mean time to complete the PSS-I was 21.96 min (SD = 1 1 S l ) , and mean time to complete the CAPS was 32.75 min (SD = 15.94). The CAPS was found to take significantly longer than the PSS-I to administer, t(41) = 5.93, p c .001, Cohen's d = 0.78. When we sampled only those patients with PTSD (as indi- cated by the PSS-I; n = 16), the CAPS still took significantly longer (M = 42.76,

190 Foa and Tolin

SD = 10.74) than did the PSS-I (M = 28.69, SD = 9.92), t ( 15) = 4.64, p < .001, and the effect was greater than before (Cohen’s d = 1.36). Thus, the PSS-I ap- pears to be a briefer instrument than the CAPS, and this is particularly true for interviewees reporting significant PTSD symptoms.

Discussion

Results of the present study suggest that the PSS-I compares favorably to the CAPS, as evidenced by internal consistency, item-total correlations, intersubscale correlations, and interviewer-rater agreement. In terms of validity, the total score and subscale scores of the PSS-I correlate strongly with the corresponding scores on the CAPS. When the PSS-I and the CAPS are used to predict PTSD diagnosis according to the SCID, both the PSS-I and the CAPS show moderately strong agreement with the SCID. Results for the CAPS vary according to the scoring rule used; however, in general, it appears that the PSS-I may have slightly higher sensitivity, whereas the CAPS may have slightly higher specificity. Thus, the PSS- I may have a small advantage in detecting actual PTSD, whereas the CAPS’S advantage may be in ruling out false positives. However, it should be emphasized that differences between the CAPS and the PSS-I were relatively small compared to their similarities.

Limitations of the present study include a relatively small sample size, com- pared to the large numbers of participants to whom the CAPS has been administered (e.g., Weathers et al., 1999). The present study examined only civilian trauma vic- tims, and thus the obtained results may not generalize to combat veterans. We did not collect data on the test-retest stability of either the CAPS or the PSS-I; such data would shed more light on the comparability of the two interviews. Finally, although interviewers were trained in both the CAPS and the PSS-I, because of the institution where the study was conducted (MCP Hahnemann University), most of the interviewers were more familiar with the PSS-I. Additional studies using interviewers who are equally familiar with the CAPS and the PSS-I would help to clarify this issue.

Because the two instruments show such similar internal consistency, inter- viewer-rater reliability, and validity, the PSS-I may be a useful alternative to the CAPS. In this study, the PSS-I took significantly less time to administer, with no appreciable loss of psychometric strength. Thus, when time and/or financial resources are limited, the PSS-I may be the interview method of choice for the assessment of PTSD.

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