In statistical analyses of simulated insecticide efficacy trials it was found that changes in distribution of target insects caused by reductions in abundance may result in incorrect inferences. The severity of the problem is directly related to the index of aggregation from Taylor's power law. Thus, highly aggregated species, such as many pests, are likely to be inconsistently sampled, with the result that insecticide efficacy trials may suggest an incorrect conclusion with far higher probability than the nominal significance level. Transformation of the data and the use of measures of mortality such as Abbott's method do little to alleviate the problem. A simple enumeration of test reports inInsecticide&Acaricide Tests(Entomological Society of America, College Park, Md.) suggests that a large proportion of tests in the literature may be erroneous, some completely incorrect.