51 The Otology Questionnaire Amsterdam: a generic patient reported outcome measure about the severity and impact of ear complaints 3 variable and was chosen to measure frequency and impact of complaints because it enables us to quantify data that is difficult to quantify.6,9 A 5-point Likert scale ranging from ‘(almost) never’ to ‘(almost) always’ was used to quantify the frequency of complaints. A 5-point Likert scale ranging from ‘strongly disagree’ to ‘strongly agree’ was used to quantify the impact of complaints. Medical students and people without any medical knowledge discussed the content of OQUA-v2 to identify questions that were unclear or ambiguous. Based on these results, items were reformulated by our expert panel which resulted in OQUA-v3. D. Pilot study 2. After substantially adjusting the questionnaire, it is recommended to perform another pilot study in a new sample of the target population (again 15-30 persons).6 Hence, OQUA-v3 was pilot-tested (N = 39) to assess the comprehensibility of the reformulated items using think-aloud interviews. Items that were perceived unclear or ambiguous were reformulated in OQUA-v4. Phase 2: definite selection of items. Field-testing of OQUA-v4 was conducted (N = 352) to perform quantitative analysis. The number of completed questionnaires required to perform proper field-testing equals seven times the number of items on the questionnaire.10 Quantitative analysis consisted of the following steps: A. Missing values (missings) and distribution of the scores. First, the percentage of missings for each item was determined. Items with <3% missings were considered acceptable and items with >15% missings were considered unacceptable.6 Second, each item’s score distribution was examined. The items with VAS scales were checked for a normal distribution using a Q-Q plot, histograms with a normal distribution curve and the significance in the Kolmogorov-Smirnov test. For items presenting normally distributed scores, the mean and SD were calculated to examine the distribution of the scores. For items presenting non-normally distributed scores, the median and interquartile ranges were calculated.6 The distribution of the scores of items with a Likert scale was examined by determining how frequently each response option was selected. B. Inter-item correlation. An inter-item correlation matrix was created. Items with low correlations (<0.2) with any of the other items were removed. Items correlating highly (>0.9) with any of the other items were checked for being identical.6 C. Explorative factor analysis (EFA). EFA was conducted to gain insight into the factor structure of the OQUA and to examine possibilities for item reduction. EFA
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