Standard Deviation

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

               5= 60/√n

              σ x¯ = 10/2= 5

              √n = 60/ 5

             n= 144

thus the  sample  size  will include  144 units for 95 percent confidence level with mean  expendture of ± Rs. 10.

                              √n = s/σ x

                            √n= 80/5

                           n= 256

thus the addition  of 112 units will be a useful  an appropriate sample  size  for the  consequent  researcher.