The standard deviation of samples (s) can be estimated by the pilot study past studies and the ranges of distribution. A pilot study may be conducted and the estimated standard deviation of the pilot sample may be used to determine the size of the sample. The previous studies already conducted may also prove to be a guideline to determine the sample size. The range of distribution with the interval in mind the standard devotion may be calculation of standard deviation. With the interval in mind the standard deviation may be estimated by dividing the range by 6 as practically all values at 99.7 percent of confidence will include the mean ± 3 times the standard deviation of the population. We can illustrate this with the standard deviation of past studies. If the standard deviation s(s) assumed to be 60 and the acceptable error is estimated at ± Rs.10 the sample size at 95 percent confidence level will be.
σ x¯ =s/n
2σ x¯ =10
σ x¯ = 10/2= 5
√n = 60/ 5
thus the sample size will include 144 units for 95 percent confidence level with mean expendture of ± Rs. 10.
√n = s/σ x
thus the addition of 112 units will be a useful an appropriate sample size for the consequent researcher.