Naive observers are found to estimate numbers of dots on projected slides with a consistent factor of underestimation of ca. 0.75. There is an overall difference in response between males and females and between more experienced and less experienced observers. There is an apparent trade-off between accuracy and precision when significant differences in response are recorded. Training observers with slides of revealed value alone is equally effective as a combination of slides and a reference pamphlet: both produce improvements in accuracy and precision. A subjective visual index of house fly abundance is found to be more sensitive than Scudder grill counts, spot cards, sticky tape traps or square-foot panels to changes in house fly population densities. None of these methods is consistently the best correlated to the visual index, although the square-foot panels were most highly correlated most often. / Ph. D.