@ Qasim, it is based on the relative number of total points scored by each alga present in a particular water body. It was developed for rating of water bodies as moderate, high and low organically polluted. According to Palmer's Algal Pollution Index values between 0-10 indicate lack of organic pollution, 10-15 moderate pollution, 15-20 probable high organic pollution and 20 and above as confirmed high organic pollution.