The type of substrate on which the bloodstain is deposited also influences TSD estimates . Substrates range from common materials like glass, plastic and textiles to less common materials like soil , wallpaper , leather and brick stones . The same applies to environmental conditions; warmer environments catalyze biomolecular degradation while colder environments preserve the biomolecules in their native states, slowing the degradation . A variety of environmental conditions have been studied in bloodstain TSD research; this includes exposure to outdoor environments , varying temperatures (-20 to 61 °C) and a wide range of humidity values (10 to 99% RH) . Statistical analyses also differ between studies; regressions are common, but the type of fit varies (linear , polynomial , logarithmic , exponential , multivariate 39 ) depending upon the observed time-series trends. A number of studies used dimensionality-reduction tools for their analyses while others incorporated cross-validation to obtain prediction accuracies . The inter-study variability in experimental conditions and statistical analyses raises two fundamental questions: how do we interpret all this research together and what is the best way forward for bloodstain TSD research?