Validating a new scale
The aim of the study was to perform a comprehensive literature review to enable the practices in PRO primary psychometric validation studies to be listed and described, with a particular focus on the importance on sample size determination. Because the focus was on primary studies, we excluded studies that reported translation and transcultural validation, revised scale validation and scale revalidation. Measure of a patient reported outcome PRO Report of a scale construction and evaluation of its psychometric properties primary study Published in English or French Published from January to September Abstracts available on PubMed Report of psychometric properties validation Exclusion criteria were: Construct validity The ability of an instrument to measure the construct that it was designed to measure. This is exemplified by its use in a growing number of published guidelines including: Extraction of data from articles, and filling in a reading grid and providing a synthesis. Our motivation was to examine how developers of new PRO measures currently determine their sample size, and report the critical steps of psychometric validation of their newly developed instruments, in terms of design, measurement properties, and statistical methods. In order to proceed with a manageable data extraction phase, in terms of time and available resources, whilst keeping the data representative of the literature, a sample of articles was randomly selected Additional file 1 AF1 , using the sample function in R 2. This allowed the judgement consistency related to inclusion and exclusion of articles to be measured, in both the pre-selection step, and the subsequent selection step [ 18 ].
For these instruments to be useful in clinical research, they must undergo a proper psychometric validation, including demonstration of cross-sectional and longitudinal measurement properties. Conclusions The sample size determination for psychometric validation studies is rarely ever justified a priori. Descriptive statistical analyses mean, standard error and frequencies for each variable of the extraction reading grid were performed. Construct validity The ability of an instrument to measure the construct that it was designed to measure. Convergent validity Involves that items of a subscale correlate higher than a threshold with each other, or with the total sum-score of their own subscale [ 2 ]. PRO measurements can facilitate patient involvement in decision-making about their own care, and may help healthcare professionals to identify patients concerns. Future health status is considered as a better indicator than the true value or a standard [ 2 ]. Known group validity The ability of an instrument to be sensitive to differences between groups of patients that may be anticipated to score differently in the predicted direction [ 2 ]. Statistical analysis To evaluate whether the reviewers agreed with each other, the proportion of observed agreements P0, and the Kappa were computed. Because the focus was on primary studies, we excluded studies that reported translation and transcultural validation, revised scale validation and scale revalidation. Once articles were pre-selected by title, the same procedure was used to score the available abstracts, using the same article selection and disagreement resolution process. Nevertheless, sample size is only briefly mentioned in most guidelines published in order to help researchers design studies aimed at assessing PRO psychometric properties, or evaluating the methodological quality of those studies [ 1 , 9 - 11 ]. Because the use of searchable technical terms for indexing international literature in databases is not always up-to-date, we defined a search strategy composed of free text terms, synonyms and MeSH terms with high sensitivity but low specificity. Search strategy The primary inclusion and exclusion criteria were chosen to meet the objective of the study: This quantitative evaluation requires a study to be conducted on an appropriate sample size. It can be divided into concurrent validity and predictive validity [ 2 ]. Prior to using PRO measures in clinical practice or research, instruments need to be developed and validated cautiously, in order to avoid biased results that might lead to incorrect interpretations. This is exemplified by its use in a growing number of published guidelines including: Agreements between authors were assessed, and practices of validation were described. The variables of the grid were discussed among the authors and yielded 60 variables in 5 domains general information on article, study and scale, determination of sample size, items distribution and evaluation of psychometric properties to describe the reporting of studies in terms of design, measurement properties and statistical methods Additional file 2 AF2. Face validity The ability of an instrument to be understandable and relevant for the targeted population. Identification of articles by specifying inclusion and exclusion criteria, keywords and search strings in the PubMed database. Article pre-selection by reading titles, followed by a selection by reading abstracts. Although inappropriate sample size can lead to erroneous findings in many aspects of PRO development and validation, in particular the identification of the correct structure of the questionnaire eg. Concurrent validity The association of an instrument with accepted standards [ 2 ]. There were two kinds of disagreements: Extraction of data from articles, and filling in a reading grid and providing a synthesis.
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