Building on applications in anthropology, the use of stable isotope ratio analysis of human bone collagen to investigate an individual’s life history is becoming more commonplace in the forensic science community. Careful consideration of the resultant isotope delta (δ) values, particularly in regard to their accuracy and reliability, is paramount when introducing isotope data into the U.S. court system. In this study, we use a simple framework to calculate real interpretative difference (RID) values for collagen (“col”) and assess isotope data comparability for sample analysis (RIDanalysis) as well as sample preparation/analysis combined (RIDcombined). The RIDcombined values of 0.59‰ for δ13Ccol and 0.91‰ for δ15Ncol are similar to more complex, published calculations of inter-laboratory variability in the stable isotope analysis of skeletal remains, but they are easier to calculate and intuitively elegant. The RIDcombined as well as RIDanalysis values presented here allow users to examine multiple sources of inter-laboratory isotopic variation (preparation, analysis, and both together) in a two-step  process whereby a RID value is constructed and then tested. Implementation of this RID approach will provide surety for the legal and research communities in forensic applications of stable isotope ratio analysis.

Here we present a two-part study to assess isotope data comparability between two sample preparation laboratories, based on the concept of Real Interpretative Differences (RID). Inter-laboratory isotopic variability should be evaluated prior to compiling data from different sources. Isotopic variability can result from two main causes: sample preparation and sample analysis. We compared the carbon and nitrogen isotope delta (δ) values of 20 modern human bone collagen sample pairs—representing seven individuals from diverse backgrounds—thatwere prepared close in time at two laboratories and then analyzed at one facility. From this study, values for RID of preparation, or RIDprep, were calculated for bone collagen as 0.43‰ and 0.75‰ for δ13C values and δ15N values, respectively. These RIDprep values were tested using a set of 30 bone collagen sample pairs—representing 30 modern individuals—that were first prepared over a span of five years (2014–2019) at one laboratory; they were prepared again at the second laboratory in 2019. While the differences in isotope δ values between the paired samples in the second study were statistically significant, they did not exceed the calculated RIDprep values for either carbon (30 comparisons) or nitrogen (30 comparisons)—with the exception of one carbon difference in a single paired bone sample. The inherent error rate in combining the isotope test results was thus calculated as 1.7%. This study demonstrated that the isotope data generated from human bone collagen prepared at one laboratory can be reliably compared to data generated from samples prepared at the other laboratory.