This article examines aggregation bias in price index construction. Specifically, we test whether changes in values of 2- to 4-unit, multi-family rental housing properties vary systematically in the same market across property size. Moreover, we examine the time trend differences across locations within a geographic region for various sized multiplex properties, as well as investigate how size should be measured. Results suggest that absolute price changes are significantly different across property size, as determined by living area, and that the time trend does not differ across locations within a geographic region. Further research using this methodology is recommended for other property types.