Statistical Treatments of Data

The AMC Robust Statistics Toolkit was used to provide a calculation, similar to one-way ANOVA, to estimate the underlying within- and between-group standard deviations. Only data from laboratories returning nine or ten results were included.

Results and trends of the five ILCs

The following graphs summarise the performance of laboratories over the course of five rounds of ILC. Blue bars represent the within-laboratory standard deviation of red bars the between-laboratory standard deviation.

 

Trends in δ¹³C results from ILCs 1-5. Blue bars represent within-laboratory standard deviation, red bars represent between-laboratory standard deviation.

Following ILC #3 large standard deviations both within and between laboratories were reported for a sample of horse hair. This was attributed to the existence of different morphologies within the sample, presumably with different isotopic signatures.

Trends in δ¹⁵N results from ILCs 1-5. Blue bars represent within-laboratory standard deviation, red bars represent between-laboratory standard deviation.

Results for nitrogen isotopic analysis have, generally, improved over the course of the five ILCs. The exception being the sample of ammonium nitrate distributed as part of ILC #5. This sample also proved problematic for hydrogen and oxygen measurements. This sample provides the tallest red bar in both charts.

Trends in δ²H results from ILCs 1-5. Blue bars represent within-laboratory standard deviation, red bars represent between-laboratory standard deviation.

Results for hydrogen isotopic analysis have, generally, been poorer than for other elements. A particularly problematic material was the sample of ammonium nitrate distributed as part of ILC #5, producing the tallest red bar in the chart. The sample of cellulose distributed during the same ILC yielded much more encouraging results.

Trends in δ¹⁸O results from ILCs 1-5. Blue bars represent within-laboratory standard deviation, red bars represent between-laboratory standard deviation.

The sample of ammonium nitrate distributed as part of ILC #5 proved problematic and produced the largest between-laboratory SD observed to date. The large between-laboratory SD observed for the bone sample distributed as part of ILC #5 results from laboratories analysing difference fractions for the bone (carbonate or collagen) and to confusion regarding the reporting scale.