The measurement process often involves an aggregating operation that has not been systematically studied by sociologists. Sometimes one aggregates “similar” behavioral acts of the same individual, whereas on other occasions “similar” individuals are aggregated. But similar with respect to what? If the aggregated units are similar with respect to underlying causes or assumed effects, then the rationale for aggregation is relatively clearcut, though complications will usually be encountered. But if aggregation has been based on administrative convenience or geographic proximity, it may be exceedingly difficult to make meaningful micro- versus macro-level comparisons. The so-called problem of “ecological correlations” is merely a special case of this difficulty, which is a fundamental one for sociological research and theory. The causal approach to measurement error may be utilized to conceptualize the problem, but ultimate resolutions will require much better data than are presently available.