to determine a sample must know the character of the samples whether homogeneous or heterogeneous. if the sample character quite homogeneous sample is a sample of 30-50. if the character of a heterogeneous sample, the number of samples of 50-100 samples. good sample should be representative of and proportional corresponding part number.
if a population is not known, it can use a technique Zikmud that the number of samples ranging from 96 respondents.