An evaluation of sampling methods used to produce insect growth models for postmortem interval estimation

Abstract

Many authors produced carrion insect development data for predicting the age of an insect from a corpse. Under some circumstances, this age value is a minimum postmortem interval. There are no standard protocols for such experiments, and the literature includes a variety of sampling methods. To our knowledge, there has been no investigation of how the choice of sampling method can be expected to influence the performance of the resulting predictive model. We calculated 95 % inverse prediction confidence limits for growth curves of the forensically important carrion flies Chrysomya megacephala and Sarconesia chlorogaster (Calliphoridae) at a constant temperature. Confidence limits constructed on data for entire age cohorts were considered to be the most realistic and were used to judge the effect of various subsampling schemes from the literature. Random subsamples yielded predictive models very similar to those of the complete data. Because taking genuinely random subsamples would require a great deal of effort, we imagine that it would be worthwhile only if the larval measurement technique were especially slow and/or expensive. However, although some authors claimed to use random samples, their published methods suggest otherwise. Subsampling the largest larvae produced a predictive model that performed poorly, with confidence intervals about an estimate of age being unjustifiably narrow and unlikely to contain the true age. We believe these results indicate that most forensic insect development studies should involve the measurement of entire age cohorts rather than subsamples of one or more cohorts.

MALDI-TOF MS as a Novel Tool for the Estimation of Postmortem Interval in Liver Tissue Samples

“…These signs continue to be the main basis for estimating the PMI 1 . For many years, various approaches have been used to determine the PMI, including examinations of thanatochemistry 2, 3 , DNA/RNA degradation 4, 5 , and forensic entomology 6, 7 . However, these methods estimate the postmortem interval using a few—or even just one—specific parameters, and their precision and time-frame of applicability are often limited.…”

The Role of a PMI-Prediction Model in Evaluating Forensic Entomology Experimental Design, the Importance of Covariates, and the Utility of Response Variables for Estimating Time Since Death

“…This is an elementary aspect of good design for many kinds of experiments [17], and we examined the implications for a carrion insect age prediction model [18]. We were motivated by the fact that some authors deliberately collected biased samples by targeting the largest larvae in a single-age cohort, and by the fact that authors who claimed to take a random sample did not describe any randomization method [18], without which they could not have sampled even approximately at random [19].…”

“…We were motivated by the fact that some authors deliberately collected biased samples by targeting the largest larvae in a single-age cohort, and by the fact that authors who claimed to take a random sample did not describe any randomization method [18], without which they could not have sampled even approximately at random [19]. Given that taking a random sample would require first physically isolating each individual from a rearing container, we doubt that an author who had done this would fail to mention it in the description of experimental methods.…”

“…Therefore, in most circumstances, one might as well sample (e.g., kill and measure) all insects in an age cohort. Perhaps a random sample, removing all insects in a rearing container without replacement at the chosen age followed by a randomization procedure to select a subset for data collection, might be worthwhile if the measurements were particularly time-consuming and/or expensive [18]. We think it is clear that the most commonly used sampling scheme in carrion insect development studies, i.e., repeatedly removing a small number of larvae from a rearing container, is a faulty experimental design and should be discontinued.…”

Research progress in the estimation of the postmortem interval by Chinese forensic scholars

“…From 1984 to 2015, Chinese forensic scholars have made great progress in improving the estimation of PMI. Many methods have been applied to help determine the time since death, and these can be divided into the following categories: molecular biology methods (degradation of DNA, RNA or proteins); spectroscopic technology (Fourier transform infrared or Raman microspectroscopy); entomological methods (either a carrion insect development or a succession model [ 3 ]); estimation of energy changes in the body after death (cooling or blood ATP levels); thanatochemistry methods (describing changes in the chemical composition of various body fluids [ 4 ]); and other methods such as imaging technology, electrophysiological methods and enzyme activity. Despite the fact that studies on the estimation of PMI span decades, there is still a long way to go before many of these can be applied definitively in forensic practice.…”