Interpretability of change in the Nurses Work Functioning Questionnaire: minimal important change and smallest detectable change

Objectives: The present study evaluates the interpretability of individual changes and assesses the minimal important change (MIC) for improvement and the smallest detectable change (SDC) of the Nurses Work Functioning Questionnaire (NWFQ). Study Design and Setting: Data of two time points, baseline and 3-month follow-up, of a randomized controlled trial were used. A total of 358 nurses and allied health professionals of one Dutch academic medical center participated at both measurement points. Separate MIC values were calculated for the total score of the NWFQ and its six subscales, using two anchor-based methods: mean change and receiver operating characteristics (ROC) curve methods. Two methods for baseline corrections were applied: subgroup analyses and MIC calculation based on relative change scores. The SDCs were calculated using the standard error of the measurement. Results: MIC values ranged from 3.4 to 8.3 for the mean change method and from 1.5 to 9.5 for the ROC curve method. In a subgroup with high-baseline scores, the MIC values of the two methods ranged from 4.4 to 29 and 9.5 to 41.5, respectively. The SDC values ranged from 7.2 to 17. Only one MIC value exceeded the SDC; however, 10 of the 14 MIC values exceeded the SDC in the high-baseline group. Conclusion: Three of the seven NWFQ scales exhibited sufficient interpretability of individual change. For four scales, conclusions on the interpretability of change cannot yet be drawn. SDCs were small compared with the scale range. (c) 2012 Elsevier Inc. All rights reserved.