Development and validation of a questionnaire to test knowledge and practices of dietitians regarding dietary supplements

Abstract 

Objective

We developed a questionnaire to evaluate dietitians' knowledge and practices about the use of dietary supplements in light of new draft regulations in South Africa that will allow dietitians to prescribe supplements for the management of nutrition-related diseases.

Methods

A cross-sectional validation study was carried out in two phases: 1) questionnaire planning and development and 2) determination of reliability (internal consistency). The study sample comprised 48 nurses, 105 dietetic interns, and 367 registered dietitians. An expert academic group comprising nine dietitians with research and teaching experience rated the academic relevance and importance of knowledge questions in an item pool of 355 questions to ensure face and content validity. Criterion validity was measured by testing the knowledge questions (KQs) on 48 nurses and 105 dietetic interns. Thereafter questions were deleted based on their difficulty, discrimination, and Cronbach's α values. The final questionnaire, which at this point included a test of 50 KQs, was mailed to registered dietitians (n = 1450) in South Africa to determine internal consistency of the final KQs.

Results

Criterion validity of the KQs was indicated by a significant difference (P = 0.000) in the mean test scores between nurses (36.9, 95% confidence interval 24.2 to 39.6) and dietetic interns (40.2, 95% confidence interval 38.8 to 41.7). Item discrimination was attained by the deletion of NQs having a minimum correlation of 0.20 with overall test score. Further, item difficulty was controlled for by removal of questions correctly answered by more than 80% (too easy) or fewer than 20% (too difficult). Internal consistency of the KQs, after completion by 367 dietitians, was 0.87 (Cronbach's α). The mean knowledge score of dietitians ranged from 56.5% to 62.5%, with the oldest group (≥40 y) having the highest score (P = 0.018). Thirty-eight percent of dietitians recommended dietary supplements at least 3 times a week; 51% consumed dietary supplements themselves; and 17% sold them to clients.

Conclusions

A valid and reliable questionnaire that tests knowledge and practices of dietary supplements was developed for dietitians. This tool can be used by academic institutions to evaluate the knowledge and practices of final-year students. It can also be used on a representative sample of dietitians in South Africa and possibly elsewhere within the profession.

Keywords: Knowledge test, Validity, Reliability, Dietary supplements