Field validation of post-mortem interval estimation based on insect development. Part 2: Pre-appearance interval, expert evidence selection and accuracy baseline data.

Abstract

This is the second part of the results from the field experiment aimed at validating PMI estimation based on insect evidence. First, we investigated the accuracy gains of PMI after supplementing age estimates with PAI. Second, we compared the impact on PMI of various ways, in which an expert selects insect evidence for the estimation. Third, we provided baseline data regarding the accuracy of PMI as estimated using different methods, insect species and life stages. Insects were sampled from nine pig carcasses exposed in a forest habitat, one carcass every one or two months, with 8-11 samples taken from each carcass using standard techniques. It was found that insect age alone clearly underestimates the true PMI across insect species and life stages. When age estimates were supplemented with PAI, the estimated PMI became generally significantly closer to the true PMI. Averaging PMI across multiple different pieces of evidence yielded more accurate estimates than using single evidence. The best single evidence method tested in this study, i.e. the use of the oldest evidence from the latest colonizing species, yielded the average PMI errors of about 22 % or 25 % (depending on the type of PAI used). For the best multiple evidence method, i.e. averaging PMI for late life stages of early colonizing species and early life stages of late colonizing species, these errors were about 13 % and 16 %. PMI ranges derived using 30 % error rate covered a true PMI in 95 % of cases but only for the best multiple evidence methods. These results demonstrate that the compound entomological method for estimating PMI (including age and PAI estimates), when applied to mock human death cases, is satisfactorily accurate. Moreover, the present findings highlight the importance of PAI and the great benefits of using multiple insect evidence to estimate PMI.