Will weak business earnings curb the results of the National Australia Bank (ASX:NAB) this week by adding to the existing pressure on its key measure, the net interest margin (NIM)?

And will any weakness in the small and medium business lending sector show up as negatives in the results next week from ANZ and Westpac, along with a further tightening in the NIM?

Most analysis has focused on bank property arrears—or potential bad loans from home lending—with a couple of analysts wondering if the rise in SME bankruptcies reported recently by ASIC could be a bit of an early warning signal for problem loans in this area.

Additionally, analysts say that while construction has been a problem area for two years—with retail as a runner-up—mining and resources, especially in WA, could very well emerge either in the interims or in the full-year figures later in the year after the collapse in lithium and nickel prices.

That's why there are forecasts for the trio to report weaker interims in the next week—starting with NAB on Thursday.

The Commonwealth showed a crimping in its revenue and NIM in its half-year report in mid-February, and that is expected to be repeated by NAB, ANZ, and Westpac.

Consensus forecasts have NAB reporting a cash interim of $3.553 billion, and Goldman Sachs, for example, reckons NAB will reveal a 13% fall in cash earnings to $3.513 billion.

A year ago, NAB reported cash earnings of just over $4 billion and statutory earnings of $3.97 billion.

The interim dividend was 3 cents a share—market consensus is for that level to be held in Thursday's report.

Goldman and some other analysts reckon that could fall to 80 or 81 cents a share. Both decisions would be a sign of the bank wanting to keep shareholders happy.

NAB's capital (CET1) was strong at just over 12%, and the key net interest margin (NIM) rose 14 points to 1.77%, or 21 points ignoring the 7-point impact from holding high levels of liquid funds in the bank's Treasury division.

Despite analysts talking about business and home loans and arrears levels, the NIM is the measure investors will be watching, followed by the cost-to-income ratio. If revenue is flat, that could be higher than expected if costs are not controlled.

The cost-to-income ratio was 42% a year ago, thanks to a 19.3% jump in revenue (and a big 11.6% rise in costs—both linked to the purchase of Citi's Australian business).

That distortion is now in the comparative base of a year ago, which will make this Thursday's figures look lower than they are.

But costs are another area investors should look at—ignoring the impact of the Citi purchase, costs were up around 6% a year ago, and while inflation has slowed, it does remain sticky.

The CBA's December 31 interim saw a 3% dip in revenue reported and a 3% dip in cash earnings to just over $5 billion. Most analysts think that's the guide for NAB, ANZ, and Westpac.

The CBA delivers a third-quarter update on May 9 (Thursday week).