Construct Validity and Validity to Change of the Patient-Specific Functional Scale in Patients with Shoulder and Low Back Pain: A Clinimetric Study

Research Article

Phys Med Rehabil Int. 2021; 8(2): 1181.

Construct Validity and Validity to Change of the Patient-Specific Functional Scale in Patients with Shoulder and Low Back Pain: A Clinimetric Study

Kromer TO1,5, Saner J2,5, Sieben JM3 4,5 and Bastiaenen CHG4,5,6*

1Faculty of Health, Safety, Society, Furtwangen University, Germany

2School of Health Professions, Institute of Physiotherapy, Zurich University of Applied Sciences, Switzerland

3Department of Anatomy and Embryology, Maastricht University, The Netherlands

4Research Line Functioning & Rehabilitation, Maastricht University, The Netherlands

5Caphri Research Institute, Research Line Functioning & Rehabilitation, Maastricht University, The Netherlands

6Department of Epidemiology, Maastricht University, The Netherlands

*Corresponding author: Caroline H.G. Bastiaenen, Associate Professor, Research Program Functioning and Rehabilitation, Department of Epidemiology, P. Debyeplein 1, 6229 HA Maastricht, The Netherlands

Received: May 28, 2021; Accepted: July 09, 2021; Published: July 16, 2021


Background: Patient-specific and condition-specific measures are widely used in clinical practice and research to measure disability or change over time. While condition-specific outcome measures comprise a range of restrictions generally relevant for all patients, the Patient-Specific Functional Scale measures restrictions chosen by the individual patient.

Objectives: Based on the hypothesis that patient-specific and conditionspecific scales deliver comparable results when used on group level. The aim of this study was to test for floor and ceiling effects, to evaluate construct validity and validity to change of the Patient-Specific Functional Scale when compared to condition-specific outcome measures. For this purpose, two datasets from patients with shoulder pain and low back pain were analyzed.

Methods: Patient-Specific Functional Scale scores were compared to the Shoulder Pain and Disability Index and the Roland Morris Disability Questionnaire at 4 time-points using stem-and-leaf-plots and correlations using Pearson’s r. Hypothesis-driven correlation levels for data interpretation were predefined, with r ≥0.75=high, r ≥0.5=moderate, r ≥0.25=low.

Results: Patient-Specific Functional Scale floor effects were comparable to condition-specific outcome measures in both samples. At none of the timepoints did the Patient-Specific Functional Scale correlate with the conditionspecific outcome measures in the expected manner.

Conclusion: Hypotheses regarding expected ranges of correlation between the Patient-Specific Functional Scale and the condition-specific outcome measures for construct validity and validity to change were not met. While the use of the Patient-Specific Functional Scale in a clinical context has its advantages, the measure is not recommended for group-level evaluations.

Keywords: Patient-centered outcome; Validity; Subacromial pain syndrome; Low back pain


BL: Baseline; CSOM: Condition-Specific Outcome Measures; GPE: Global Perceived Effect Scale; LBP: Low Back Pain; MCI: Movement Control Impairment; NDI: Neck Disability Index; NRS: Numeric Rating Scale; ODI: Oswestry Disability Index; Pearson’s r: Pearson’s Correlation Coefficient; PSFS: Patient-Specific Functional Scale; RMDQ: Roland & Morris Disability Questionnaire; ROC: Receiver Operating Characteristics Curve; SD: Standard Deviation; SPADI: Shoulder Pain and Disability Index; SPS: Subacromial Pain Syndrome; T1-T3: Follow-up Time Points.


For a therapist, it is essential to ascertain whether improvements in body function or structure also lead to increased activity and participation levels. Therefore, the use of assessment tools which can reflect the actual status or degree of restriction and which can measure patient’s change over time is of crucial importance. Improvements in body functions and structures are predominantly assessed through physical testing; activities and participation are commonly measured using questionnaires. Scores gathered using these measurement tools also allow comparison at a group level and enable patients, therapists and researchers to “measure” the impact of a disease, the progression over time or the effect of an intervention. However, since questionnaires often contain very specific items related to certain activities, it is possible that some items will not be relevant to all patients in the target group. By that, the importance of the individual items could vary between patients. Moreover, a “prefixed” item set may not include activities that are of importance to individual patients. Therefore, patients may be required to score questions that are only partly relevant to them. As a result, these standard questionnaires might not adequately reflect a patient’s individual restrictions or the change in these restrictions over time. In an attempt to solve this problem, the Patient-Specific Functional Scale (PSFS) was developed with the intention to monitor a patient’s progress based on relevant restrictions chosen by the individual himself [1]. The PSFS is comprised of 1 to 5 activities; each activity is rated on an 11-point Numeric Rating Scale (NRS) from 0 (impossible to do) to 10 (no difficulties at all). The PSFS is easy to administer, and takes about five minutes to complete. However, the PSFS also has been used in the past by researchers to determine the current state of function and the development of activity restrictions over time on an average group level. By choosing this approach, researchers have moved away from the originally intended individual focus of the instrument and applied the PSFS to situations for which it was not developed or validated. From a test-theoretical perspective, there are numerous problems in deviating from the original construct. Firstly, the interpretation of an average score across self-selected activities by individuals is a challenge. For researchers and clinicians who are familiar with interpreting data on a clearly defined aspect of disability, it is tempting to interpret outcomes using the same approach; but in fact, one is averaging different constructs. Another problem is that floor or ceiling effects could occur if a patient chooses either lightly activities with scores at the lower end of the scale or severely restricted activities with scores at the upper end of the scale. In the first case, it is difficult to detect a positive development and in the second case to detect a negative development over time; this may affect results for validity to change analysis to a certain degree. Problems may also occur when the initially chosen activities become increasingly irrelevant as a problem as time gone by, due to either the patient’s improved condition or reduction in complaints or because of seasonal effects, when the activity becomes more and more irrelevant during follow up as for example snow shoveling in spring. Dependent on the activities chosen by the individual patient it could also be that outcomes in the PSFS indicate higher or lower disability levels for that patient compared to Condition-Specific Outcome Measures (CSOMs) and that scorings on the PSFS may differ significantly more between patients than their corresponding outcomes on a CSOM, where all patients rate the same standardized set of items. Despite these problems, which have not yet been adequately realized or addressed, several researchers have investigated the psychometric properties of the PSFS on a group level for a variety of musculoskeletal conditions. Results have been formulated as “promising”, since the PSFS has been reported as having good construct validity, discriminant validity, and responsiveness [2-4]. Based on these results we think that testing psychometric properties and comparisons at a group level can be justified by defining the PSFS as an instrument assessing “activity restriction based on items selected by an individual patient” as the overarching construct. We hypothesized that specific musculoskeletal disorders (in our example subacromial shoulder pain and low back pain) lead to specific activity restrictions and specific pain patterns. CSOMs in our case the Shoulder Pain and Disability Index (SPADI) and the Roland & Morris Disability Questionnaire (RMDQ), summarize these typical activities and include a range of tasks from easy to more difficult. These questionnaires were designed to include items that cover the whole range of items assumed relevant for a patient group, although not every item may be of equal importance to each individual patient. Therefore, we assume that many activity restrictions chosen by individual patients for their PSFS can be traced back or are closely related to items listed in the CSOMs. If this is the case, the PSFS could be approached as a construct and, because of the hypothesized close association between the operationalization of both types of measurement, assumed to deliver a relatively high correlation with the CSOM, especially in a cross-sectional analysis. However, there is also the possibility that the PSFS measures a different dimension not covered in the COSM. Taking these arguments into consideration, the aims of this paper are threefold: In two groups of patients, suffering from either Subacromial Pain Syndrome (SPS) or Low Back Pain (LBP), and using the CSOM as an external standard comparator: 1) To test for possible floor and ceiling effects of the PSFS; 2) To evaluate its construct validity compared to the CSOM, and 3) To assess the ability of the PSFS to detect changes over time with reference to an external anchor [5].


Data were used from two different datasets collected during randomized controlled trials investigating effects of physiotherapy interventions in a patient group with SPS and a second group with LBP in primary care. A detailed description of the inclusion processes, applied treatments and primary analyses can be found in the published study protocols [6,7] and trial results [8-11]. Ethical approval was granted by the ethics committee of the Ludwig- Maximilians-University Munich, Germany (project-no. 018-10) for the SPS trial, and the Swiss Ethics Committee granted ethical approval (KEK-ZH-NR: 2010-0034/5) for the LBP trial. All patients in each trial gave informed consent. Datasets of the two samples were analyzed independently of each other.

Dataset 1 - SPS patients

Participants were recruited through referral for physiotherapy due to shoulder complaints. After baseline assessment, they were randomly assigned to either an intervention or a control group. The intervention group received exercise therapy plus manual therapy, while the control group received only exercise therapy. Baseline characteristics of the 90 participants included in the trial are presented in Table 1.