Link Function for Binomial Model in Estimating Knockdown Time (KT95 and KT50) of Mosquito Repellents

Estimation of knockdown time (KT) is useful in determining bio-effectiveness of mosquito repellents. Knockdown or not knockdown is a binary variable thus, analysis is done by fitting generalized linear models, based on binomial distribution. Use of appropriate link function in fitting a generalized linear model is crucial especially when estimating quantities such as KT 50 and KT 95 . This study was done to determine the most appropriate link function in fitting generalized linear models to estimate KT 50 and KT 95 . Knockdown activity of metofluthrin 0.005% (w/w) and d-trans-allethrin 0.12% (w/w) was tested under two different physiological conditions (blood fed and sucrose fed) using wild-caught female Culex tritaeniorhynchus mosquitoes from an agro-farming area of the north-western province of Sri Lanka. Coefficient of variation of the observed KT50 and KT95 was less than 5.5%. Both KT 50 and KT 95 values were estimated by fitting altogether 120 binomial distribution-based generalized liner models with three different link functions namely, logit, probit, and complementary log–log. The G2 statistics was used to test the goodness of fit of the models. However, in order to evaluate the accuracy of all estimated KT 50 and KT 95 values obtained using the above three link functions, they were compared against corresponding observed values using ANOVA followed by Dunnett mean separation procedure. The probit and logit link functions were found to be appropriate in the estimation of KT 50 . As the logit link function is commonly used in modeling binary responses, out of the two, logit link function is recommended. Complementary log–log link function was found to be the most appropriate in estimation of KT 95 . Thus, one link function cannot be recommended in estimating both KT parameters. Tropical Agricultural Research Vol. 25 (3): 396-402 (2014)