Radio stations are to discover for the first time exactly how many listeners are tuning in - or turning off - after agreeing to sponsor a joint ratings survey. The multi-million dollar study, announced yesterday, will enable channels to schedule programmes more effectively and help advertisers target audiences more accurately. Metro, Commercial and RTHK, which have produced their own figures in the past, agreed to co-operate on the independent survey following more than five years of negotiations. The decision was welcomed by the Association of Accredited Advertising Agents of Hong Kong and the Hong Kong Advertisers' Association. The survey of 28,000 people will be split into two to establish listening patterns during and outside the racing season. Market researchers A. C. Nielsen will begin the first batch of 12,000 interviews next Monday. The second phase will be in mid-September.