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The Posttraumatic Stress Disorder Checklist (PCL)
The Posttraumatic Stress Disorder Checklist (PCL) is self-report assessment for measuring posttraumatic stress disorder (PTSD). The PCL consists of items that target traumatic experiences and PSTD symptomology that follow from those experiences based off of the Diagnostic and Statistical Manual for Mental Disorders (DSM). The PCL-5 is the most recent version of the PCL, basing its items off PTSD criteria from the Diagnostic and Statistical Manual for Mental Disorders 5th Edition. This measure was developed by the National Center for PTSD and is currently in the public domain.

Brief Background
Posttraumatic Stress Disorder (PTSD) became a formal diagnosis by name in 1980 in the 3rd edition of the Diagnostic and Statistical Manual for Mental Health Disorders (DSM-III). However, during this period of time, few formal assessments of PTSD symptomology were formulated. The National Center for PTSD designed the first PCL (PCL-4) in 1993, a year before the formal publishing of the new DSM, the DSM-IV (Weathers et al, 1993). Since its creation, it has become one of the gold-standard assessments for PTSD along with the Clinician-Administered PTSD Scale (CAPS) and has been utilized in both clinical and research contexts.

Format of the assessment
The PCL has always served as a self-report measure of mental health symptoms to aid in the diagnosis and treatment of PTSD. The PCL total number of items have varied depending on the version of the assessment used, though all versions of the assessment have a similar format. The instructions of the assessment detail that each item the subject is meant to complete reflects symptoms that people can experience after exposure to a traumatic event. It then requires subjects to answer questions to indicate the level of disturbance the symptoms bring about for them, and give a temporal restriction on experiences (“In the past month”). Items are directly related to symptoms of PTSD, such as the first item: “Repeated, disturbing memories, thoughts, or images of a stressful experience from the past?” Response scales of the PCL vary across versions, with the most recent version on a 0-4 Likert scale (0= Not at all to 4 = Extremely). The basic PCL assessment often takes around 7-10 minutes to complete, with variability of timing influenced by the respondent as well as the format given (e.g. clinical interview).

When scoring and interpreting the assessment, the National Center of PTSD highlights several scoring methods: a total summed score of all items, summed scores based off the individual symptom clusters (e.g. intrusive symptoms, avoidance, etc.),  and utilizing each item in relation to its diagnostic criterion counterpart in the DSM-5 by considering any item that is scored with a 2 (moderately) or higher as a symptom being present. If the total score of all items is utilized as a scoring method, research conducted by a variety of researchers recommend that a PCL-5 score greater than 33 suggests a positive screening for PTSD as a provisional diagnosis. As typical treatment requires repeated assessment to determine if symptomology has been altered by treatment, the National Center of PTSD also gives general guidelines to note when change occurs: a PCL-5 total score change of 5-10 points indicates statistically significant change, and a PCL-5 total score change of 10-20 indicates a statistical and clinically significant change.

The PCL-4
The PCL-4 was first published in 1993 by the National Center for PTSD in Boston, MA, and corresponds with diagnostic criteria for PTSD delineated in the DSM-IV. As such, the criteria the items are based off of are similar to that of the DSM-5, but without the mood/cognition component. This version of the PCL assessment has 17-items and came in multiple formats to be used with various specific populations: a civilian version (PCL-C), a military version (PCL-M), a version for symptoms related to specific stressors (PCL-S), and a version for parental report of PTSD symptoms in children (PCL-PR). According to the National Center for PTSD, the most recent PCL-5 corresponds most with the PCL-S version of the PCL-4, and there are currently no updated versions of the PCL-C and PCL-M. The response scale of the PCL-4 items are from 1 (Not at all) to 5 (Extremely) in response to how bothered the respondent has been by the symptom in question in the last month.

The PCL-5
The PCL-5 was designed by Weathers and colleagues in 2013 as a version of the assessment that was congruent with diagnostic criteria for PTSD based in the DSM-5. As such, the PCL-5 has items congruent with the different symptom domains of PTSD based on the DSM-5: Exposure to a traumatic event (directly experiencing, witnessing, etc.), intrusive symptoms (Distressing intrusive memories, nightmares, flashbacks, etc.), avoidance of stimuli associated with trauma, negative mood and cognitive changes (e.g. memory difficulties, low mood), as well as reactivity and arousal changes (e.g. hypervigilance, recklessness, etc.). This version of the PCL has three different formats: the original assessment with items around PTSD symptomology only, a version of the assessment that includes qualitative questions related to Criterion A of PTSD (exposure to a traumatic event in some form) and PTSD symptomology items based on prior responses, as well as a clinical interview format. The response scale of the self-report items are from 0 (Not at all) to 4 (Extremely) in response to how bothered the respondent has been by the symptom in question in the last month.

Reliability & Validity
A predominance of psychometric literature on the PCL has been conducted on the PCL-4 due to the recency of the development of the PCL-5. Researchers evaluating the reliability and validity of the PCL-4 noted that the assessment has demonstrated internal consistency in a variety of different populations exposed to traumatic experiences, including victims of sexual violence and military veterans, across more than 20 studies. The PCL-C has demonstrated good reliability and validity characteristics overall, as well as utility and accuracy in military populations with careful consideration of cutoffs. However, reviewers of the PCL have noted that the assessment is susceptible to the influence of variability in PTSD symptom presentations and bias that may influence the diagnostic accuracy and validity of the PCL. Reviewers also noted that the PCL has often been used as an assessment of PTSD prevalence in studies seeking to validate the measure itself, which is counter to its utility as a screener that requires a priori understanding of prevalence to be evaluated effectively. Though a majority of literature has focused on the psychometrics of the PCL-4, literature on the PCL-5 exists and also reflects good reliability and validity.