THE Police Complaints Committee is to try to discover why most people who lodge grievances against the force withdraw them before they are investigated. The force has received about 3,300 complaints about its officers in each of the last three years, but could not investigate two-thirds of them because those complaining refused to give further details. The proportion of cases withdrawn and closed due to lack of evidence had gradually increased from 69 per cent in 1990 to 76.9 per cent last year. It was particularly common for those claiming they were assaulted by officers to withdraw their complaints. Assault claims accounted for half of the complaints received in the past three years. The fact that most complaints were related to crime reports and crime arrests was believed to be responsible for the high back-down rate, according to the Complaints Against Police Office (CAPO). Police believe people have refused to give full details either for fear of exposing their line of defence in a future court hearing or facing prosecution upon an unfavourable court ruling. New procedures to suspend investigation of complaints pending a court ruling or legal advice were introduced to tackle the problem, but proved to be little help. ''So far, it appears just as many complainants fail to respond to our letters after the court case, or withdraw allegations, as was the situation before the new procedures were introduced,'' a CAPO quarterly report said. The chief staff officer of the Complaints and Internal Investigation Branch, Mr Jim Walker, said that it would be necessary to find out the real reasons behind the problem before seeking a remedy for the sake of improving the police complaints service.