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

the population. The estimated totals are the weighted totals of the children’s records inthe final analysis file, subset to those children who experienced the maltreatment inquestion. Incidence rates are computed by dividing the estimated total number ofchildren who experience the maltreatment in question by the number of children in theU.S. population 16 and by 1,000. This provides the rate per 1,000 children in the generalpopulation. The ensuing chapters present estimates for the incidence maltreatment asdefined by the Harm Standard and by the Endangerment Standard, giving the estimatednumbers of children who experienced the different categories of maltreatment as well asthe corresponding rates per 1,000 children in the population. Analyses also producedestimated totals and rates for subgroups of children, defined by various child and familycharacteristics. In each case, the denominator used to compute the rate reflected thenumber of children in the general population with the corresponding characteristic. 17The national estimates in this report derive from a sample of maltreatedchildren. The final NIS–4 analysis file includes a sample of 12,408 children whoexperienced Endangerment Standard maltreatment, 5,226 of whom also experiencedHarm Standard maltreatment. Any estimate based on a sample has some degree ofassociated uncertainty. Sample-based studies index the amount of uncertainty onestimates with the “standard error of estimate,” or S.E. The square of the standard erroris the variance. The standard error or variance indicates the precision of the estimate. Itis critical in understanding how reliable an estimate is and in deciding whether anobserved difference in estimated rates is “statistically significant,” or whether it couldsimply be due to sampling error (due to chance fluctuations). 18 Statisticians recommend16 In order to accommodate any changes in the size of the overall child population in the U.S. between theNIS–4 reference periods (fall 2005 and spring 2006), NIS–4 incidence rates used the average of the July2005 and July 2006 U.S. Census Bureau annual estimates as the population denominator (U.S. CensusBureau, 2008a).17 To provide estimates for the broad range of characteristics of interest, the NIS–4 used populationstatistics from various sources; subsequent chapters provide those details. In all cases, those sourcesprovided the information about the distribution of the characteristics in the child population. To establishthe specific denominators for computing rates, that distribution was applied to the NIS–4 overall childpopulation denominator (based on the average of July 2005 and July 2006 child population estimates, asdescribed in an earlier footnote). This ensured that the subgroup denominators summed to the NIS–4child population denominator.18 Statistically significant differences have a very low probability of emerging by chance (probability of 5%or less, or pp>.05).2–18