Thread: confidence level and confidence interval?

Hi,

I am currently writing my dissertation and I am in the process of analysing the data from my survery, I have been told I need to consider confidence level and confidence interval.

I have done some research and I think I understand in basic terms what they are but I have been searching everywhere but I cannot find out how to work them out or apply them. I found this confidence interval calulator:

Sample Size Calculator - Confidence Level, Confidence Interval, Sample Size, Population Size, Relevant Population - Creative Research Systems

Which is fine if i assume a 95% confidence level, i guess this is where i am getting stuck, what is my scientific justification for using a 95% confidence level?

I am a bit stuck on the whole thing so any guidance would be greatly appreciated right now...

Thanks,
Richard

I would need to know what paramter you are trying to estimate.

Basically, when trying to determine a sample size prior to sampling,
you set the standard error times the percentile, which is half the width of your interval
< your maximal error and then you solve for n, your sample size.

You can select any confidence level you like. Confidence levels are directly proportional to the size of the interval. So, for example, the confidence interval for the mean of a population at the 95% level might be between 3 and 4 while the at the 99% level it's between 2 and 5.

EDIT: The size of the confidence interval is inversely proportional to the size of the sample. If you are not able to collect a large sample, you might be better off selecting a lower confidence level.

One more thing. A 95% confidence interval does not mean that there is 95% chance that the paramater be estimated (such as mean or variance) is actually in that interval. What it does mean is if you performed many similiar experiments of the sample size from the same population, 95% of the constructed intervals would contain that parameter.