The authors correlated account fees, the minimum account sizes required to avoid fees (fee thresholds), and interest rates paid on deposits with the "generalized HHI" (GHHI), a metric similar to MHHI. They concluded that for interest-bearing checking accounts, a one-standard-deviation increase in GHHI increased fees by about 11% and fee thresholds by around 17%.
If there are two airlines servicing the market and their market shares are equivalent, HHI will be 5,000, MHHI[DELTA] will be 5,000, and MHHI (HHI + MHHI[DELTA]) will be 10,000.
Under Elhauge's proposal to police common ownership using the Clayton Act's Section 7 (which, by its literal terms, precludes stock acquisitions that tend to lessen market competition), liability would result from "any horizontal stock acquisitions that have created, or would create, a MHHI[DELTA] of over 200 in a market with an MHHI over 2500" if "those horizontal stock acquisitions raised prices or are likely to do so." Posner et al.