Statistical Applets: Confidence Intervals for Proportions
This applet simulates the process of taking an election poll involving two candidates. Some proportion p of the population plans to vote for candidate A. If we sample n people from the population, the proportion of people in the sample who say they plan to vote for candidate A is our best estimate of the population proportion. We can then construct a level C confidence interval, where C is the probability that the interval actually includes the true value of p. More specifically, the confidence interval is calculated as the sample proportion ± z* times the standard deviation of the sample proportion, where z* is the critical value of z that has (1-C)/2 of the normal distribution to the right of the value, and the standard deviation is .