Population studies on Oncomelania quadrasi, the snail intermediate host of Schistosoma japonicum, in the Philippines. 2. Necessary sample size for snail density survey.

In order to save time and manpower for the density surveys of Oncomelania quadrasi, the snail host of Schistosoma japonicum in the Philippines, the minimal necessary sample size (q) was determined using a formula, q greater than t2/E2(1d - 1 + 1/-x). This formula is based on the Id dispersion index [2, 3] which varies with the degree of clumping of unevenly distributed animals. A nomograph was prepared for the ready determination of the necessary sample size for various degrees of clumping (Id) and mean density (-x) to be encountered in the field. The reliability of the sampling procedure with an interval of 5 m distance, which has been adopted to the routine snail survey in the Philippines, was examined using the relative error (E). The relative errors calculated ranged below or around 20% (E = 0.2). Therefore, the existing sampling method was proven to be highly reliable as the field survey. Comparison was made between the sample size actually taken and the minimum needed theoretically determined by this method, by adopting the Student's t = 1 and relative error E = 0.3 as a permissible reliability level in the field. The actual sample size was more than twice the minimum needed for 21 out of 30 populations surveyed and less than the needed in 5 of the 30 surveys.