It's a key measure of a bank's profitability. The industry's average net interest margin fell to 3.43 percent in the third quarter from 3.56 percent a year earlier. Some big banks have also cautioned that their earnings are up mainly because they've shed jobs, bad loans and weak businesses rather than because of an improved economy. They include JPMorgan Chase & Co ., Bank of America Corp ., Citigroup Inc. and Wells Fargo & Co. All managed to recover from the financial crisis in part because of federal aid. Small and midsize banks have taken longer to rebound. They held risky commercial real estate loans used to develop malls, industrial sites and apartment buildings. Many such loans weren't repaid. But as the economy has strengthened, fewer such loans have soured, and many small and medium-size banks have recovered. For example, at M& T Bank Corp ., a regional institution based in Buffalo, N.Y., net income soared in the third quarter.