RT Journal
A1 Ghosh D
T1 EFfect of preventive supplementation on young children in niger
JF JAMA
JO JAMA
YR 2009
FD June 3
VO 301
IS 21
SP 2208
OP 2209
DO 10.1001/jama.2009.740
UL http://dx.doi.org/10.1001/jama.2009.740
AB 
There are 2 levels of correlation in the study that need to be accounted for in the analysis. The first is that the children were clustered within households. This is shown in Figure 1 of the article, which indicates 1671 children from 647 households in the intervention group and 1862 children from 760 households in the control group. This leads to use of mixed-effects models in which correlation is accounted for at the village, household, and individual levels.