Left truncation criteria for survival analysis of white-tailed deer

Survival estimates are commonly obtained by physically capturing wildlife and marking or affixing a transmitter to a representative sample of the population. Bias induced by capture stress can occur for white-tailed deer (Odocoileus virginianus) if capture influences the probability of mortality. To mitigate this bias, researchers often left truncate data for a threshold number of days (14-28 days for deer) after capture. Potential costs of left truncation include reduced sample size and reduced inference. Costs associated with capture and monitoring of deer are substantial, and defining a truncation period is usually arbitrary or ad hoc. Hence, researchers need to evaluate objectively the effects of left truncation. We analyzed time-to-event data from 1,001 radio-collared white-tailed deer from northern forests and eastern farmlands of Wisconsin, USA in 2011-2014 to evaluate justification for using a 2-week truncation period by comparing the probability of mortality for deer <2 weeks post-capture and deer 2 weeks post-capture. We found little support for a difference in mortality between the groups. Results accounting for time of year, study area, and age suggested a 0.69 probability that deer 2 weeks post- capture had a lower mortality rate than deer <2 weeks post-capture. Using a reference group of 6-10 -month-old deer in the eastern farmlands in 2011, cumulative capture season mortality was 0.102 (50% CI=0.075-0.125; SE=0.037) for deer <2 weeks post-capture and 0.114 (50% CI=0.087-0.138; SE=0.037) for deer 2 weeks post-capture. Our results suggested that when a careful review of cause-specific mortality is considered for each deer, and presumed capture-related mortalities are removed from the sample, a 2-week truncation period is unnecessary. (c) 2016 The Wildlife Society.