ONLY 13 in every 1,000 complaints lodged against the police last year were upheld despite legislators' repeated criticism that the police were biased in handling disputes with the public. But the police-run Complaints Against Police Office (CAPO) denied the record low figure confirmed that it was slanted in favour of the police. The 1.3 per cent rate of proven complaints last year was a further decline of 0.1 per cent on the 1992 rate. Last year's rate is the lowest since 1986, when the Police Complaints Committee (PCC) was set up to monitor CAPO. Under the present system, CAPO carries out investigations which are ratified by the separate PCC with independents appointed by the Government. The figures, to be released in the forthcoming PCC annual report, showed a downward trend in the number of successful allegations. In 1987, the rate was 8.9 per cent. It was 3.6 per cent in 1990, 2.3 per cent in 1991. CAPO's Senior Staff Officer Senior Superintendent Ross Williams said the final figure for the proven rate could be higher as there were 1,033 complaints lodged last year still under the office's scrutiny. Mr Williams admitted however that it was understandable if the public cast doubts on his office, which was part of the 27,000-strong police force. ''It's natural that the public do harbour some reservation,'' he said. ''All I can say is that we do investigate every case objectively. ''And we do ask searching and cogent questions in order to know the truth.'' He said the proven rate should not be used as a yardstick to measure the office's impartiality. ''You cannot judge CAPO's effectiveness by the number of cases proven, that's ridiculous,'' he said. ''If the evidence isn't there, we can't say an allegation is substantiated.'' He said some of the cases were settled through the introduction of an informal resolution scheme, which could reduce the rate of justified complaints. The scheme, which covered minor offences such as abusive language, allowed cases to be settled through discussion in the presence of a superintendent at a police station. He also pointed out that CAPO had been manipulated by some criminals trying to disrupt prosecutions put forward by the police before the court. ''People do use us and occasionally, people do make false complaints,'' he said. Mr Williams admitted that CAPO had to be cautious and prove beyond reasonable doubts on serious allegation like assault. He said his investigators could take the principle of ''balance of probability'' in cases of minor offences. Legislators voted last April for CAPO to turn independent in order to boost public confidence. But the call was rejected by the Government, which instead introduced measures to provide more transparency to the complaints unit. The measures included allowing PCC members to interview witnesses and including lay observers during CAPO's investigation process.