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Chapter 9168please the surgeon, or to fill out the form rapidly just before an appointment. In the postoperative data collection, many questionnaires were given and this could have resulted in questionnaire fatigue, making the obtained results less reliable. In our studies OHIP-14 forms were handed out in person by the surgeon or by a dental student. This can also influence the results and the willingness to fill out the questions. A trial with automatically sent questionnaires, using the Lime-Survey program, showed even less response, although 3 reminders were sent at regular times. All the factors above will influence research outcomes, and the power analyses used to design a study should reflect this.Another consideration in the use of OHRQoL as an outcome measure is addressing and measuring clinically meaningful change. Statistical significance is used to demonstrate the importance of results, but sample and size variation within studies play a very important role in determining statistical significance. While significant results showing pre/post group change may be appropriate for use in population-based health policy, they may not be appropriate for clinical care outcomes or clinical trials measuring within-group effects.7, 8Larger numbers of participants can reduce the overall bias, but a prolonged study time is necessary. This makes the study more susceptible to failure due to unforeseen events in the study group—for example, researcher or student illness, closure of the institute or new legislation affecting research—making completion of the study and thus publication of its results more challenging. The advantage of the 7-days postoperative questionnaire was that patients returned to the outpatient clinic after a week for a control visit and handed over their questionnaires. The biggest challenge of the 1-year orthognathic study was to collect all the OHIP-14 questionnaires at all the time points. To our disappointment, only 22 of the 85 patients completed all the questionnaires. This could be because the patients were operated by different surgeons and received preoperative orthodontic treatment from various orthodontists. In the future, the response rate might be improved by using a different approach to collect the data. If patients were approached by just one study surgeon/researcher, personal familiarity with them might make the patients more committed to filling out all the OHIP-14 questionnaires at all time points. To acquire more information on the OHIP-14 status of nontreated patients, a future study should include patients newly visiting the orthodontist, so that the preoperative orthodontic treatment, which involves realignment of the