In early stage drug discovery screens, the efficacy of a potential drug candidate in an in vivo assay is often judged, rather informally, on the proportion of animals that respond to the drug. Only if at least a prespecified proportion of animals respond is the drug considered active enough for further testing. This is a "simple screening strategy." Researchers are often unaware of the properties of such strategies and what effects changing the parameters of the strategy, such as the sample size, could have. It is suggested and demonstrated here that the properties of a simple screening strategy can be easily understood by considering its true and false positive rates.