For all analyses, univariate analysis was performed first to examine the effect of each variable separately. Multivariate analysis was then performed by including all variables with P values less than 0.20 in univariate analysis and exclusion of variables with P values greater than 0.10, using a standard backward likelihood ratio method. For all continuous variables, we also presented categorical results in 3-5 categories to allow inspection of the data and determine whether or not the assumption regarding continuous variables (quadratic for temperature) was justified. Statistical analyses were performed using the Stata package (StataCorp LP, College Station, Texas) (20) . Readers interested in further research can contact the corresponding author to obtain the full data set used in this study.