Detection, identification, and correction of a bias in an epidemiological study

The relative lack of epidemiological studies of natural populations is part ly due to the difficulty of obtaining samples that are both large enough an d representative of the population. Here, we present the result of an epide miological study (December 1992-August 1995) of feline immunodeficiency vir us (FIV) in a free-roaming population of domestic cats (Felis catus), with a special emphasis on sample bias. Over five trapping periods, the prevalen ce of FIV in sampled cats steadily declined. Across these samples we consis tently achieved a very large sampling fraction (approximately 60% of the po pulation), the sex ratio, age and weight distributions remained stable with time in the samples, and the sex ratio was similar in the samples and the population. These indices would normally indicate that our samples were rep resentative, Suggest study of whole population revealed an important bias i n the samples, with an initial high probability of capturing a few individu als, which appeared significantly more likely to be FIV-infected, and then a lower probability of recapturing them. Since our protocol resulted in a n on-random sampling, subsequent trappings were designed to avoid this bias, by also capturing individuals who had previously learned to escape capture. This modified capture regime revealed that FIV prevalence was in fact cons tant in the population. This study shows how samples of large size, which a re stable and appear representative of the population, can still be biased. These results may have major implications for other studies based on trapp ing.