The table, above, presents aggregate figures for the SDM 100 group of companies, which are ranked by their recurring monthly revenue—an industry standard of valuation of a security monitoring business. Most of the SDM 100 firms are privately held. Submitting recurring monthly revenue is required for ranking; other figures are not required but mostly provided. Most firms —but not all —also reported their total annual revenue, number of subscribers, and installation revenues. Therefore, one should exercise caution in using this information to extrapolate industry totals or to benchmark. Note: approximately seven companies that reported substantial residential sales revenue in 2008 did not report residential sales at all for 2009; therefore, it appears that residential took a drastic fall. In reality, if those firms are omitted from the calculation, residential sales were just about even with the prior year.