What Should be the Sample Size?

It is a fundamental question of any data based research. A big sample  ensures accuracy and wider predictability of the results. But more time and money get involved in this process making it expensive and time consuming. This might not be realistic for countries like Nepal.In comparison to a developed country, research is conducted here with much less financial support from governmental funding agencies. So if inherent variability of the population with respect to the objective of study can be known in advance from some previous studies, a small sample that is big enough to reflect this variability can be equally efficient. So while deciding about the sample size a compromise has to be made. Here time/money is on one side and accuracy is on the other side. So if inherent variability is less, the population is homogeneous and a small sample will be enough to reflect this variability. But for heterogeneous population, the sample size should be big enough to reveal this variability.