Hi

Universe of 2110 documents have to be sampled. Using the sample calculator, with 95% confidence level and 4% Margin Error the sample size arrived is 467.

However, when I selected randomly 10% sample (211 documents), the sample mean and sample median is not too much deviating from the population mean and population median. The SE Mean is also very low (0.00787).

Sample Mean - 2:08 (h:mm) - 2:07 is population mean.

Sample Median - 1"04 (h:mm) - 1:04 is population median

CI for Mean = 1:45 (h:mm) and 2:30 (h:mm).

CI for Median = 0:47 (h:mm) and 1:16 (h:mm)

My question is, considering this example, if 10% sample can have good representation of the population then how significant is the 467 sample size which came out of the sample size calculator of 95% CL and 4% CI? I am not sure if this question is limited to the example I have, but wanted to understand the concept.

Please help.

Thanks

Madhan

Hi Madan,

If you simply consider population size and confidence level (CL) you can only calculate confidence interval (CI). Hence your calculation of 467 sample size tells that you can be 95% certain population picks the defined mean value within the

confidence interval.

If I have understood your question correctly, with your 10% sample your mean anyway is same or very close to the population and why do we need 467 samples. Why not 211.

It's quite possible all your data points are very close to the mean which means variation is quite low.

But if you take any random sample without calculating, in this case let's say 211 not 467 then you need to be clear about one more factor called percentage. Accuracy also depends on the percentage of your sample that is how close to a particular value/answer. In this case population mean. What I see is nearly 90% of your sample (211) are closed to population mean. The chances of error are remote, irrespective of sample size. However,

if the percentage is 50%, which means 50% of your 211 are away from population mean,  the chances of error are much

greater.  In your case if you change your percentage to 50% you will get CI of 6.4 for 211 sample size.

I hope this explains.

Regards

Jyotiram