Results on the frequency% distribution of indexed PDFs show that no major difference occurs from operator to operator; the average standard deviation on measurements between the three operators being only 1.2%, with a maximum standard deviation of 4.1%. For the relative proportion of unindexed planes, observer ability had an influence, as only ~7-8 relative% of the planes measured by two experienced operators were unindexed, whereas the inexperienced operator reported 12-15 relative% unindexed planes. For each section, number of PDF sets per grain varies from operator to operator. However, this variation, which does not exceed ~20%, is not directly influenced by observer experience and is probably related to localized sample heterogeneity or unintentional selection of more heavily shocked grains. This study shows that it is critical that published PDF orientation histograms clearly define what frequency measurements are used, i.e., relative or absolute frequency, whether or not unindexed PDF sets are included in frequency calculations, and numbers of grains and plane sets analyzed. This is essential for comparing datasets from different studies or from different workers examining the same samples.
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