After presenting Commonwealth Bank's results for the six months to December, which featured strong growth in income, a low bad debt charge and modest expenses growth, chief executive Ian Narev was asked what factors might make it difficult for the bank to sustain its performance.Narev nominated three things: bad debts, markets' income and wealth management investment performance.Impairment costs, as a proportion of loans, are below the bank's long-term average and might get worse if employment numbers continue to weaken. The markets component of the institutional banking division's earnings during the half was strong. Narev said this business tended to be volatile because of factors the bank could not control.And, in the wealth management division, 89 per cent of the group's managed funds produced returns that were above their relevant benchmarks. Above-average returns tend to attract funds inflow but it is hard to sustain such a high level of performance.CBA reported a net profit of A$4.2 billion for the six months to December - an increase of 16 per cent over the previous corresponding period and an increase of six per cent over the result for the six months to June. On a cash basis, after adjusting for non-recurring items, the bank reported a net profit of $4.3 billion - an increase of 14 per cent over the previous corresponding period.One of the most significant aspects of the results was the reduction in the charge for bad and doubtful debts, which fell 26 per cent on the previous corresponding period, to $457 million. Loan impairment expense, as a proportion of gross loans and acceptances, has fallen from 73 basis points in 2008/09 to 16 basis points in the latest half.Narev also described the sort of "black swan" events his risk management team included in its scenarios. "The number one concern would be a rapid slowdown in China," he said."We look at what would happen if global markets freeze and if the Australian economy goes into a period of stagflation, where inflation and interest rates are high and economic growth is low."However, Narev said, the bank's strong performance was a result of the continuation of its long-term strategy of improving customer satisfaction, improving productivity and investing in technology. He is confident this strategy will continue to produce results.