Clustered data: The term applied to both the data in which the sampling units are grouped into the clusters sharing some common feature, for instance families or geographical regions animal litters, and longitudinal data in which a cluster is de?ned by the set of repeated measures on the particular unit. A distinguishing feature of such type of data is that they tend to exhibit intracluster correlation, and their analysis requires address this correlation to reach valid conclusions or result. Methods of analysis which ignore the correlations tend to be inadequate enough. In particular they are likely to provide the estimates of the standard errors which are too low. When the observations contain the normal distribution, random effects models and the mixed effects models might be brought in use. When the observations are binary, giving rise to the clustered binary data, suitable methods or techniques are mixed- effects logistic regression and the generalized estimating equation approach.