Comparing bank profits, even with relatively neutral measures such as return on equity, can be a bit of a minefield taking into account changes in accounting practice, shifts in classification of supposedly one-off items and the melange that is "cash profits".Still, with NAB having rounded off the reporting season for major banks yesterday it is worth taking stock.The following comparisons are all drawn from data compiled by KPMG, which continues to collate this data for the benefit of journalists and others.Using the statutory profit first, the aggregate profit for ANZ, CBA, NAB and Westpac over the first half of their 2010 financial year was $14.0 billion, up from $9.7 billion in the second half of the 2009 financial year and $12.5 billion in the first half of 2009.The median return on equity for the big four in the current half, using the slightly higher cash profits as the numerator, was 15.7 per cent.This compares with returns on equity of around 18 per cent for the major banks in 2005 (under the old accounting standard) and more than 19 per cent in 2006 (under AIFRS).A couple of banks briefly boasted returns that exceeded 20 per cent in the years preceding the credit shock.Given the need by all banks to raise fresh capital and dampen dividends over the last three years, returns on assets probably provides a better comparison.Westpac and Commonwealth Bank are the most profitable, with ROAs of 0.96 per cent and 0.93 per cent respectively, based on the first half 2010 financials. ANZ's ROA is 0.76 per cent and NAB trails the pack with an ROA of 0.64 per cent.APRA data shows returns on assets ranged around 1.0 per cent to 1.2 per cent in the years preceding the credit shock, so the two best performing of the major banks are close to the low end of that range.The impairment charge is the key swing factor in bank profits at present - all the more so given that asset growth is slight while revenue growth is also subdued (partly due to no asset growth and partly due to eliminating one common class of fees). For the four banks the impairment charge was $4.5 billion in aggregate in the first half, down from $6 billion or more in each of the last two halves.While bank managements have attempted to push a message of moderate improvement, impairments continued to worsen at all banks over the last half. Most of the trouble might have been recognised and pushed within their institutional businesses, but the bad loans keep piling up within the middle market business segment and below.ANZ reported the largest rise in the level of impaired assets over the half, up 20 basis points to 1.56 per cent, and ANZ's level of impairments is also the worst. NAB is not far behind with a ratio of 1.44 per cent.Commonwealth Bank's and Westpac's loan quality are much the same, and reported by the banks to be a basis point or two under one per