"Are the PO Invoices + Non-PO Invoices supposed to add up to the Paper Invoices + Electronic Invoices? If I take the average for the 5000-19,999 companies, for example, I get 756 + 969 = 1725 for PO+Non-PO; but 1,360+468 = 1828 Paper+Electronic. Why do these not add up to the same amount? I'm off by about 100 invoices, and I can't understand why."
The sums of the averages and medians for PO and Non-PO invoices will not equal the respective sums of the averages and medians for paper invoices and electronic invoices. While it may seem as though they should, they do not because:
1) Not all companies have all types of invoices, i.e. PO and non-PO, paper and electronic. For example, some companies may have PO and non-PO invoices but all are paper, with no electronic invoices. A few companies may have a strict PO-only policy and therefore have no non-PO invoices, while others may not use POs at all. For a given company-size category, averages and medians are calculated across all companies in the particular category. Not all categories will have the same number of companies because not all companies have data in all categories. (We do not include "0"s in averages.)
2) We throw out the low and high for each data series prior to calculating averages and medians, in order to exclude anomalies at the extremes. (This is a standard statistical analysis practice.) These lows and highs do not occur in the same companies from category to category. This also causes differences in the data outcomes in the different categories.