The credit quality story continues to improve in the industry, with a marked improvement for Commonwealth Bank disclosed in its quarterly earnings over the three months to September 2015.Troublesome and impaired assets reduced to A$5.5 billion, the bank said yesterday in a trading update. This is down from $6 billion at June 2015 and is close to half the level three years ago.The bad expense was $220 million over the quarter, a fall of $30 million over the quarterly average over the last year.This trend helped Commonwealth Bank produced one of its best quarterly earnings over the quarter to September 2015, with a profit of A$2.4 billion on a cash basis and a statutory net profit of $2.3 billion.CBA said "income growth was similar to FY15 while underlying group net interest margin was stable."