Background Over time, the circumstances encountered in case of child maltreatment, can be quite complex and then, can lead to methodological questions for the analysis of the data. Based on data coming from 395 children hospitalised, alone (66.1%) or in siblings (33.9%), in a paediatric ward between 2007 and 2012 for mistreatment or because of a severe risk of mistreatment, the aims of this study were to quantify the degree of similarity between sibling members, to study the differences between children hospitalised alone or with siblings and to compare four statistical methods for the analyses of the associated factors of mistreatment.

Methods To quantify the degree of similarity between sibship members, the Snijders and Bosker intracluster correlation coefficient was calculated and the statistical methods used were the logistic regression and GEE, both without and with robust standard error.

Results Almost all intracluster correlation coefficients were large, meaning that the sibling’s members have a higher degree of similarity between them. The odds ratios were not exactly the same between the two models and the robust standard errors where almost always higher than the model-based standard errors in both logistic and GEE models leading to wider confidence intervals.

Conclusions Because many of the intra-siblings correlations observed were relatively strong, the failure to take this cluster dependency into account had a substantial effect on the statistical analyses. Methods taking into account the cluster dependency are widely available in statistical software and strongly recommended.