A unit of analysis error occurs when the units used in the analysis of the results of a study (e.g. individuals) are different from the units of allocation to the treatment comparison groups (e.g. clusters). If the results of a cluster-randomized trial are incorrectly analysed, as though the unit of allocation was individual participants, not clusters, this can result in an overestimation of the effect of a treatment. This type of error can also occur in studies where participants are allocated to treatments that are then applied to parts of them (for example, to both eyes or to several teeth), or if repeated observations are made on a participant. If the analysis is by the individual units (for example, each tooth or each observation) without taking into account that the data are clustered within participants, then a unit-of-analysis error can occur.
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