PLUS-M™ Frequently Asked Questions (FAQs)

Below we provide answers to frequently asked questions (FAQs) regarding the use of PLUS-M™ instruments in clinical practice and research. Please submit any other questions to us at info@plus-m.org.

General Questions
Selection Questions
Administration Questions
Scoring Questions
Interpretation Questions


General Questions

The Prosthetic Limb Users Survey of Mobility (PLUS-M™) is a self-report instrument for measuring mobility of adults with lower limb amputation.  PLUS-M™ is intended for use in clinical practice and research.

Short forms are subsets of questions in the PLUS-M™ item bank. These subsets are compiled into short surveys to offer flexible administration options. Two PLUS-M™ short forms (the PLUS-M™ 7-item short form and the PLUS-M™ 12-item short form) are available. All items on the PLUS-M™ 7-item short form are also included in the PLUS-M™ 12-item short form. For information on selecting short forms for clinical or research purposes, please see Selection FAQs and the PLUS-M™ Users Guide (see "Selecting PLUS-M™ Instruments").

Computerized Adaptive Tests (CATs) are electronic versions of standardized instruments (surveys) that selectively administer items to a respondent based upon their responses to questions. As such, CATs offer an optimal combination of high measurement precision and low administrative burden. The PLUS-M™ CAT can be administered through the PLUS-M™ Computerized Administration System and can be customized (i.e., the CAT stopping rules can be adjusted) to suit the purposes of the administrator. Calibration parameters for the PLUS-M™ CAT are also available to administer PLUS-M™ instruments using third-party software like the NIH Assessment Center. Please contact us at info@plus-m.org for more details regarding the availability and use of the PLUS-M™ calibration parameters.

PLUS-M™ short forms are free for non-commercial use. Examples of non-commercial use include administration of paper surveys in clinical practices for the purposes of monitoring patients or administration in research for the purposes of assessing study participants. For more information, please see PLUS-M™ Terms of Use.

The PLUS-M™ item bank and short forms have been rigorously developed using modern psychometric methodology. PLUS-M™ short forms may be used confidently in clinical practice and research. Evidence of PLUS-M's validity and reliability may be found in under Publications and Presentations.

PLUS-M™ instruments were developed by researchers in the Department of Rehabilitation Medicine at the University of Washington. Data to develop PLUS-M™ instruments were collected from more than 1300 people with lower limb loss with the assistance of numerous industry, hospital, academic, and clinical partners.

Prosthetic Limb Users Survey of Mobility (PLUS-M™) short forms were developed under funding from the National Center for Medical Rehabilitation Research (NCMRR), National Institute of Child and Human Development (NIH grant number HD-065340, PI: Hafner). Mode of administration equivalency testing and test-retest reliablity testing are being conducted with support from the Orthotics and Prosthetics Education and Research Foundation (OPERF, OPERF Grant Number OPERF-2014-SGA1, PI: Hafner).

We encourage citation of PLUS-M™ short forms as follows:

PLUS-M™ 7-item Short Form Citation: Prosthetic Limb Users Survey of Mobility (PLUS-M™) 7-item Short Form (insert version). http://www.plus-m.org. Accessed on [insert date].

PLUS-M™ 12-item Short Form Citation: Prosthetic Limb Users Survey of Mobility (PLUS-M™) 12-item Short Form (insert version). http://www.plus-m.org. Accessed on [insert date].

PLUS-M™ Computerized Adaptive Test Citation: Prosthetic Limb Users Survey of Mobility (PLUS-M™) Computerized Adaptive Test (insert version). http://www.plus-m.org. Accessed on [insert date].

PLUS-M™ Users Guide Citation: Prosthetic Limb Users Survey of Mobility (PLUS-M™) Short Forms Users Guide (insert version). 2013. http://www.plus-m.org. Accessed on [insert date].

The PLUS-M™ Users Guide and PLUS-M™ short forms have been translated into German, Swedish, and UK English for administration to respondents who speak these languages. Please see the translations page for details on the recommended use and psychometric properties of translated instruments.

Selection Questions

Selection of a PLUS-M™ instrument should be based on the importance of the decision(s) that will be made from the obtained T-score(s). The greater the consequences of the decision to be made, the more important it is to select an instrument with greater measurement precision.  The PLUS-M™ Computerized Adaptive Test (CAT) or PLUS-M™ 12-item short form is recommended in situations where mobility is a primary outcome (e.g., comparative effectiveness studies) or when important treatment decisions are to be made (e.g., prosthetic component selection). The PLUS-M™ 7-item short form provides adequate measurement precision (and lowers the respondent and administration burden, relative to the 12-item short form) in situations where mobility is a secondary outcome (e.g., epidemiological studies) or when patients’ health is being monitored (e.g., outcomes databases). Additional recommendations for selecting PLUS-M™ instruments may be found in the PLUS-M™ Users Guide (see "Selecting PLUS-M™ Instruments").

PLUS-M™ instruments have been purposefully developed to be used with prosthesis users. Use of PLUS-M™ instruments with persons who are not prosthesis users is not recommended.

Persons with bilateral amputation participated in PLUS-M™ focus groups and cognitive interviews, but were not included in the original sample that was used to develop scoring for PLUS-M™ instruments. We have tested PLUS-M™ instruments in more than 200 people with bilateral lower limb amputation and found no evidence of statistically significant differential item functioning (DIF) by type of amputation (i.e., unilateral vs. bilateral). This means that PLUS-M™ instruments may be used with people with bilateral amputation. However, it is important to note that PLUS-M™ v1.x T-Scores are centered on information obtained from persons with unilateral amputation, so the mean of 50 represents the mean mobility of the sample with unilateral amputation. The PLUS-M™ Users Guide has been updated with reference data from a sample of individuals with bilateral amputation (see "Bilateral Reference Sample"). We will update PLUS-M™ instruments and scoring guides on the PLUS-M™ website as additional evidence of validity in persons with bilateral amputation becomes available.

PLUS-M™ instruments have been designed to quickly and accurately measure respondent's perception of their prosthetic mobility. While PLUS-M™ instruments may be considered as an alternative or complement to other self-report measures of mobility, they are not intended to directly replace performance-based measures like the AMP. We recommend use of both self-report and performance-based measures to assess mobility.

Administration Questions

PLUS-M™ short forms require about 1-2 minutes to administer. The PLUS-M™ Computerized Adaptive Test (CAT) takes about 1 minute to administer and score (as scoring is performed automatically).

PLUS-M™ short forms require about 1-2 minutes to score. The PLUS-M™ Computerized Adaptive Test (CAT) is scored automatically. Scoring tables are included with PLUS-M™ short forms. Full instructions for scoring may be found in the PLUS-M™ Users Guide (see "Scoring Complete PLUS-M™ Short Forms" and/or "Scoring Incomplete PLUS-M™ Short Forms").

PLUS-M™ short forms may be administered electronically. Our data to date suggests there are no significant differences in PLUS-M™ T-scores obtained via paper and electronic surveys. Formal mode of administration testing has not yet been performed. We will post updates here on the PLUS-M™ website as evidence of PLUS-M™ mode of administration equivalency becomes available.

Computerized versions of PLUS-M™ instruments are now available. Please see the PLUS-M™ Computerized Adaptive Testing (CAT) page for details on how to administer the PLUS-M™ CAT.

  • Is PLUS-M™ available in other languages?

PLUS-M™ short forms are available in English (US), German, Swedish, English (UK), Farsi, Spanish, Norwegian, and French Canadian. Please see the PLUS-M™ Translations page for details on PLUS-M™ short form translations.

Scoring Questions

Full instructions for scoring PLUS-M™ short forms may be found in the PLUS-M™ Users Guide (see "Scoring Complete PLUS-M™ Short Forms" and/or "Scoring Incomplete PLUS-M™ Short Forms"). The PLUS-M™ Computerized Adaptive Test (CAT) is scored automatically and a clinical report is generated upon completion.

Instructions for scoring incomplete PLUS-M™ short forms may be found in the PLUS-M™ Users Guide (see "Scoring Incomplete PLUS-M™ Short Forms").

Scoring a PLUS-M™ short form will produce a PLUS-M™ T-score. Raw scores, which are obtained by summing responses to each question, should only be used to look up PLUS-M™ T-scores using tables provided in the PLUS-M™ short formsOnly PLUS-M™ T-scores should be reported

PLUS-M™ T-scores are standardized scores with a mean of 50 and a standard deviation (SD) of 10. A higher PLUS-M™ T-score represents a higher level of mobility.  T-scores are comparable across all PLUS-M™ instruments. This means that a PLUS-M™ score obtained by a respondent using the 7-item short form may be compared directly to a score obtained by a respondent using the 12-item short form. A PLUS-M™ T-score of 50 is equivalent to the mean score reported by more than 1000 lower limb prosthesis users included in the PLUS-M™ development study (see PLUS-M™ Users Guide).

Interpretation Questions

A higher PLUS-M™ T-score corresponds to greater mobility. PLUS-M™ T-scores are referenced to the PLUS-M™ development sample (n=1091 lower limb prosthesis users) described in this manual.  A T-score has a mean of 50 and a standard deviation (SD) of 10. A PLUS-M™ T-score of 50 represents the mean mobility reported by the development sample.  A respondent that receives a T-score of 60 has reported a level of mobility approximately 1 standard deviation above the mean. Conversely, a respondent that receives a T-score of 40 has reported their mobility to be about one standard deviation below the mean.

Individual PLUS-M™ T-scores may also be compared to those reported by the development sample or to those reported by specific subgroups in the development sample (reported by level of amputation, etiology of amputation, gender, and age). For more information on interpretation of PLUS-M™ T-scores with respect to the development sample, refer to the PLUS-M™ Users Guide.

Efforts to estimate clinically-meaningful changes in PLUS-M™ T-scores are currently underway. Estimates of measurement error (i.e., standard error of measurement [SEM] and minimum detectable change [MDC]) were obtained in a study of 199 lower limb prosthesis users indicates PLUS-M™ instruments have the following estimates of measurement error:

Instrument ICC SEM MDC (90% CI) MDC (95% CI)
PLUS-M™ CAT* 0.92 2.79 6.42 7.65
PLUS-M™ 7-item short form 0.95 2.02 4.69 5.59
PLUS-M™ 12-item short form 0.96 1.93 4.50 5.36

* Estimates of measurement error based on default CAT settings
(i.e., minimum items = 4, maximum items = 12, SE threshold = 3.0)

(Source: Hafner BJ, Morgan SJ, Askew RA, Salem R. Psychometric evaluation of self-report outcome measures for prosthetic applications. J Rehabili Res Dev. 2016; 53(6):797-812)

Changes in PLUS-M™ T-score that exceed these error estimates can be considered indicative of difference and/or change.