Estimating LD50s without a computer.
Quantitative analysis of dose-related effects, such as mosquitoes killed by insecticide or parasites killed by a drug, usually involves estimating the dose which kills, on average, 50% of the subjects. This quantity is often termed the LD(50) (LD for lethal dose), or the ED(50) (ED for effective dose). Other specified response levels, such as the LD(90) - the dose that kills 90% of subjects - may also be derived. Dose-related effects of this type follow an S-shaped curve because, clearly, doses lower than those giving zero response will also give zero response, while at the other end of the curve, doses above those giving a maximum response can also only give a maximum response. In other words the curve flattens out at both ends. The mathematics of fitting a suitable S-shaped curve to such data - for example by probit analysis - is quite simple in principle but can be arduous and time-consuming without a suitably programmed computer. In this article, Michael Healy explains an alternative approach which is particularly applicable to field observations where computers are unavailable.
London School of Hygiene and Tropical Medicine, Keppel Sreet, London WC1 E7HT, UK.
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