Commonwealth Bank's credit quality may be on the improve, with its loan impairment expense ratio markedly less than six months ago.CBA, in a quarterly trading updates, said its loan impairment expense was A$322 million in the September 2016 quarter, equal to 18 basis points of gross loans.This compares with 19 bps in the year to June 2016.Six months ago the bank said this ratio was 23 basis points over the six months to March 2016. In each of 2014 and 2015 the comparable metric was 16 bps.Across all four big banks, impaired assets to total loans and advances remained at 0.4 per cent, PricewaterhouseCoopers observed in an analysis of the reporting season for major banks.CBA said in its consumer book the LIE was higher in the quarter, "primarily driven by continued stress in areas of Western Australia and Queensland impacted by the mining downturn."The bank's cash earnings for the September 2016 quarter were approximately $2.4 billion. Statutory net profit on an unaudited basis was also approximately $2.4 billion, CBA said.