Characterization of the whole population based on sample groups will always have a representation error, which will change with growth of the sample group size. At a certain point it will be negligible and with further sampling beyond that point the error remains practically unchanged. A graphical method is proposed to identify that point, determining minimum size of the bat sample group needed to provide reliable parasite indices. Calculations with various numbers of microbat hosts and subsequent visual inspection have shown that to obtain a reliable Abundance Index of bat fleas (Siphonaptera, Ischnopsyllidae) or bat flies (Diptera, Nycteribiidae) in winter-spring season only 35-40 bats should be examined. In summer, with many young animals in populations, the number of examinations should be increased up to 50-55. Similar results are obtained for the Infestation Index. Examination of only the necessary number of hosts allows disturbance and stress levels to be minimised, as well as time and amount of field work, without negative impact on the data reliability. This is especially important while dealing with rare and endangered species.