Waqas - The number of units in a finite population is denoted by N which is the size of the population. In infinite population, sometimes it is not possible to count the units contained in the population. This is called as infinite or uncountable population.
In order to calculate the minimum sample size, you can consider the standard normal deviation set at 95% confidence level (1.96), percentage picking a choice or response (50% = 0.5) and the confidence interval (0.05 = ±5). The formula is:
n = z 2 (p)(1-p)
c 2
Where:
z = standard normal deviation set at 95% confidence level
p = percentage picking a choice or response
c = confidence interval interval for your estimation
or
You need to carry a pilot study to get some information about population , as standard deviation and use the following formula :
Sample size ll be based on your popolation size. Further, determine the same using sample size determination formula. There are many of it. Here is one,
(S)/1+[(S-1)/population]
Where S= Zsquare* p(1-p)/M square
Here S=sample size for infinite population
Z= Z score
p= population proportion( generally assumed as 50% or .5)
M= Margin of error (generally assumed as 5% or .05)