Fourth National Incidence Study of Child Abuse and Neglect (NIS–4)

APPENDIX DWITHIN STUDY ANALYSES This appendix describes the analyses and presents the statistics for the within-studyanalyses of the NIS–4 results. This introduction discusses the computation and assessment ofthe t-statistics used to identify subgroup differences within the NIS–4. The tables that followprovide the specific results of all statistical tests that support the findings presented in the NIS–4Report to Congress.All within-study statistical differences were assessed by the t-statistic on ratemeasures. To address the fact that the NIS–4 data derive from a complex sample design(stratified and multi-stage), the computations were performed using WesVar (Westat, 2007).This program calculated the estimated rates (r 1 and r 2 ) and (using the replicate weights in theNIS–4 analysis data file) the standard error of these rates (SE r1 and SE r2 ). The t-statistic wasthen calculated as follows:√r 1 -r 2t =2SE 2 r1 + SE r2The analyses examined all subgroup differences in maltreatment rates in eachcategory of maltreatment and for each level of harm or injury resulting from maltreatment. Inorder to adjust for the higher likelihood of observing significant differences when conductingmultiple comparisons in this way, the significance of the resulting t-statistics was assessed usingthe Bonferroni critical value for t (Sankoh, Huque & Dubey, 1997). The appropriate significancelevels with their corresponding alpha levels (i.e., probability of observing the result by chance)are also presented for each series of tables in the next section.ReferencesSankoh, A.J., Huque, M.F., Dubey, S.D. (1997). Some comments on frequently used multipleendpoint adjustment methods in clinical trials. Statistics in Medicine, 16, 2529-2542.Westat, Inc. (2007). WesVar 4.3 User’s Guide. Rockville, MD. Available online athttp://www.westat.com/westat/statistical_software/WesVar/pdf/WV_4-3_Manual.pdfD-i