Determination of Sample Size

Determination of Sample Size for
Estimating Means and Proportions:
l It is very important to known how large a sample
size do we need for estimating a mean or
proportion of a population that depends on:
l 1- The width of confidence interval.
l 2- The level of confidence interval.
l 3- Estimates of ?2 may be available from
previous or similar studies, or when the sample is
drawn from approximately normal distribution
population, then the estimate of ? ? range / 6.
Example: To estimate the mean weight of women use Oral
Contraceptive in family planning center. How large a
sample should be taken if we want a 99 percent confidence
interval that is 3 Kg width?
Firstly take pilot or preliminary sample from this
population, the weights (in Kg) of the 10 women who
participated in the study were:
62 53 57 55 69 64 60 59 60 60
Estimate ?2 by using s2 =20.544
n=2.582 * 20.544 / 1.52 = 61
n2=n-n1
Finally 61 – 10 = 51 is the total sample size requirement
Example: A hospital administrator wishes to estimate
the mean weight of babies born in her hospital. How
large a sample of birth records should be taken if she
wants a 99 percent confidence interval that is one pound
width? Assume that a reasonable estimate of ?2 is one
pound.
l Example: A hospital administrator wishes to know what
proportions of discharged patients are unhappy with the
care received during hospitalization. How large a sample
should be drawn if we let d = 0.05, the confidence
interval is 95 percent and no other information is
available?
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