The appropriate use of summary statistics in the Tables requires some comment. The "All Industries Sum" line refers to the sum of all the categories. The All Industries Sum ratios are ratios of these totals. The "size weighted mean" line refers to the mean (average) of the categories, weighted by the size of each category (number of firms). The size weighted mean is identical to the mean of all the firms within all the categories. This differs from the simple mean, which is the mean of the categories without regard to the different size of the categories.

The "About Simple Mean Standard Deviation" refers to the standard deviation of the categories around the simple mean. Because the information on the variation within the categories is lost in this kind of grouped data, a true measure of the variation among all firms cannot be calculated.