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The full model reduces the questionlevel variance relative to the base model by 87%, while the model with the individual question characteristics (that is, without the established tools) reduces the questionlevel variance by 86% (Online Appendix 18D, Table A18.D3). Thus, most of the questionlevel variation in RTs in these data is explained by this set of individual question characteristics. In the model that controls for the characteristics of respondents and interviewers, RTs are significantly different and longer for women, older respondents, and Latino respondents compared to other racial and ethnic groups (Online Appendix 18D, Table A18.D1); these effects remain largely unchanged in the models that also control for the question characteristics and established tools for evaluating questions (not shown). IICs by Question CharacteristicsTABLE 21.3 Median IICs by Question Characteristics (n = 102 Questions)
Source: National Health Interview Survey, 2017. ^{a} An alpha level of .10 was used to determine if differences in median IICs were statistically significant. ^{b} pValue based on a MannWhitneyWilcoxon test. ^{c} pValue based on a KruskalWallis test. (KruskalWallis test, pvalue = .056). Collapsing quartiles one through three results in a median IIC of .032 (n = 76), which is significantly lower than the median IIC for quar tile 4 (MannWhitneyWilcoxon test, pvalue = .011). We observe a similar finding for median IIC by the FleschKincaid reading ease score. Collapsing the last four categories  "standard," "fairly difficult," "difficult," or "very confusing" items  results in a median IIC of .048, which is significantly higher than the median IIC (.031) for the "very easy," "easy," or "fairly easy" items at the .10 alpha level (MannWhitneyWilcoxon test, pvalue = .085). Interviewer IICs by Interviewer CharacteristicsAs described earlier, 39 outcomes or questions were selected for the analysis of median IICs by interviewer characteristics. The overall IICs for these 39 items ranged from .0071 to .2113. Table 21.4 presents median IICs for each group of interviewers defined by each of four interviewer characteristics. For pace of interview, the fastest interviewers (group 1) TABLE 21.4 Median IICs for 39 Items by Interviewer Characteristic
Source: National Health Interview Survey, 2017. ^{л} An alpha level of. 10 was used to determine if differences in median IICs were statistically significant. ^{b} pValue based on a KruskalWallis test. ^{c} pValue based on a MannWhitneyWilcoxon test. had the highest median IIC (.049) among the three groups, with groups 2 and 3 (slowest) having similar median IICs (.034 and .032 respectively). A KruskalWallis test reveals a difference in medians significant at the .10 level (p = .073). For 22 of the 39 items, the fastest interviewers have the largest IIC. Collapsing groups 2 and 3, given their similar median IICs, and then comparing to group 1 results in a significant difference in medians at the .05 level (MannWhitneyWilcoxon test, pvalue = .023). Hence, interviewers performing at the fastest pace appear to be associated with a significantly higher median IIC compared with slower paced interviewers. Regarding interviewer cooperation rates, we observe a consistent but nonsignificant decline in the median IIC across the three groups, whereby interviewers with the lowest cooperation rates have the highest median IIC (.047) and the interviewers with the highest cooperation rates have the lowest median IIC (.020). We collapsed groups 1 and 2 into a single category. This collapsed group of interviewers had cooperation rates of approximately 88% or less and a median IIC of .040 across the 39 items. When compared to the median IIC (.020) of the interviewers with the highest cooperation rates, the difference is significant at the .05 level (MannWhitneyWilcoxon test, pvalue = .035). For the number of completed sample adult interviews measure, we observe a significant decline in the median IIC as interviewers conduct more sample adult interviews (Kruskal Wallis test, pvalue = .021). In pairwise comparisons, no significant difference in median IIC is observed between group 1 and group 2, but significant differences are observed between groups 1 and 3 (MannWhitneyWilcoxon test, pvalue = .022), and groups 2 and 3 (MannWhitneyWilcoxon test, pvalue = .013). To further underscore these findings, the smallest IIC was observed for group 3 interviewers (completed 41 or more sample adult interviews) for 28 of the 39 questions. 
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