The use of a multiple-transfer method in plant virus transmission studies- some statistical points arising in the analysis of results

The frequency of success of attempts to transmit a virus disease from one plant to another has long been used to measure the effects of any of the factors which influence transmission. Samples are taken from the population under test (e. g. vectors, diseased plants, etc.), and usually one sample is tested on each test plant; this is binomial sampling. However, in the procedure we name the 'multiple-transfer method' more than one sample may be tested on each test plant. This increases the number of samples tested without increasing the number of test plants used, and errors due to heterogeneity in the population under test are therefore minimized. Results from experiments using the multiple-transfer method may be evaluated by using the maximum likelihood estimator. The method is particularly reliable when the proportion of infected samples being studied is small, but can lead to considerable over-estimation when the proportion is high.