In order to forecast the incidence rate in the prevalence of Japanese encephalitis, in Okayama Prefecture, the authors took into consideration the following factors, average atmospheric temperature, duration of sunshines and rainfall amount in June and July as the climatic elements, date showing immunological positivity of haemoagglutination inhibiting reaction (HI reaction in short) on 50 per cent number of swine, positive rate on swine till the end of July, and the Logarithms of the numbers of Culex Tritaniorhychus till July 10th, the end of July, and August 10th.
To make smallest the mean error yielded in the case of calculating with the multiple regression equation that consisted of factors, the most important and meritorius factors were selected from various factors above mentioned (14 factors in all) in 1966-1973 by the voluntary, forward selection methods devised by us. And multiple regression equations were made of the selected factors.
When trying to forecast the incidence rate in the same prefecture, 2 factors, the Logarithms of total of C. T. involving male and female till July 10th, and the rainfall amount in June became useful. In the case of using 3 factors, the serological positivity of HI reaction on fifty per cent number of swine was added by forward selection method, and the average atmospheric temperature of June, that of July, and the rainfall amount of June became the factors by voluntary selection method. Using the multiple regression equation made from the 14 factors of 1966-1971, the incidence rate of 1973 was estimated. The results almost agree with the actual value.
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