# Public opinion poll

1. Jun 14, 2010

### Lobotomy

hello
when doing a public opinion poll before an election in a country, they usually approximate the hypergeometric distribution with a binomial distribution and then using the normal approximation:

m +/-1.96*sqrt(m*(1-m)/n) to calculate a 95% confidence interval?
m= mean
n= number of people in the poll

is it as simple as that? assuming party A gets 30% of the votes, and 2000 voted we get a 95% statistically significant interval of:
30 +/-1.96*sqrt(0.30*(0.7)/2000)

is it as simple as that? i think this is what i learned in school a long time ago...

However if we make a poll within a defined population of lets say 5000 people. and the number of people in the poll is 3000. party A gets 30% of the votes in the poll. What can we say about the total population of 5000.
here we should use the hypergeometric distribution and can not use the binomialapproximation. Ive only used the hypergeometric distribution in examples of sampling without replacement, where we want to calculate the probability of drawing a certain color ball. However this time, we draw som balls and we want to say something about the pot itself right (the balls are the votes and the pot is the total population vote). How do we do this?