At 26:00 the Hearst exponent, which helps predict the likelihood of correlated events. For example, it's much more likely to rain on Weds if it rained on Tues, too. The implication to the database, borne out by experience in the real world, is that things break together. If a hard drive goes bad in a rack, then check all the drives in the rack because they may have been part of a bad batch. So, they simulate these types of grouped failures. ^hearst-exponent-correlated-events