Unit of observation versus unit of analysis

Researchers investigated the effects of a school based educational programme aimed at reducing the consumption of carbonated drinks to prevent weight gain in children aged 7-11 years. A cluster randomised controlled trial study design was used. The programme, which was delivered over one school year, focused on promoting a healthy diet. The control group received no intervention. Six primary schools in southwest England were recruited and 29 classes were involved in the trial. Classes were randomised to treatment, with 15 classes allocated to the educational programme (325 children) and 14 to the control treatment (319 children).1 The outcome measures included consumption of carbonated drinks. Each child recorded the number of glasses (average size 250 mL) that he or she drank over a three day period at baseline and at the end of the trial, and the change from baseline was obtained. For each class (the cluster), the average change in consumption of carbonated drinks across all children was derived. The treatment groups were compared with regard to the average change within clusters. Over one school year, the consumption of carbonated drinks decreased by a mean of …