I have what is probably a fairly basic question!

I am conducting a number of surveys looking at expenditure behaviour of audiences at an event. I want to calculate confidence intervals around the figures I have got for average expenditure.

e.g. For one survey, my total population is 66

There were 28 respondents

On average, the respondents spent £100 each at the event

What I want to know is what is the confidence interval on that £100, given that the figure is based on a less than 50% sample of the population?

I have found a formula for this, in this link

http://www.dimensionresearch.com/resources/calculators/conf_means.html

But this doesn't seem to take account of the size of the population. If I put in 28 into the "sample size" box, and fill in the other stats they ask for, it gives me a certain confidence interval, but what if my population was also 28? Then there would be no sampling error. I don't see how this formula takes account of this.

Also, I have found this

http://www.surveysystem.com/sscalc.htm

However, this works on percentages and you have to input a notional % result for a question with a yes/no answer (e.g. 50% gives the highest confidence interval). This doesn't seem relevant to what I need.

There must be

*some*way of working out confidence intervals given the mean, standard deviation, the achieved sample and the population??

Can anyone help a stats-poor guy?

Cheers.