ALTHOUGH residential mortgage lending rose by 0.5 per cent in December, a slight decrease from November's 0.8 per cent, the real picture was more robust since some institutions outside the survey had recorded stronger growth. The growth figures, from $201.7 billion to $202.7 billion in December, were collected by the Monetary Authority from a survey covering 28 authorised institutions, comprising 87 per cent of the total market. ''The survey results somewhat understate the current rate of growth. This is because mortgage lending by certain institutions outside the survey group of 28 institutions has been growing quite rapidly,'' said Monetary Authority deputy chief executive David Carse. Another survey by the authority of 33 institutions covering 94 per cent of the market showed a different trend. ''It was not until last month that figures from these two surveys start to diverge. Previously the two figures displayed similar trends and growth rates,'' he said. The additional five institutions had occupied only a small share of the mortgage market. However, since the big mortgage lenders tightened credit, these smaller players had seen higher growth due to their relatively smaller base, Mr Carse said. Although these smaller players had big appetites, it did not mean they were not observing the 70 per cent mortgage lending rule. He said the authority might switch to publishing the 33-institution survey to deliver a more accurate picture of the mortgage market. ''Even so, it is clear that the overall growth in mortgage lending remained quite modest in December, probably reflecting seasonal factors as well as the banks' generally tight lending policies,'' he said. The 0.5 per cent growth in December was below the average monthly growth of 1.2 per cent over the past 12 months.