Let's suppose that you already have a confidence interval, but you want to know how many observations your sample needs in order for your interval to look that way. Or let's say that you want your interval to be a certain width: say, only 10 on both the plus and the minus side - what sample size will make sure that this is true? (The width of the confidence interval, by the way, is called a margin of error. It's what you get when you multiply the critical value by the standard deviation).
To find the sample size, set your margin of error equal to your critical value times your standard deviation:
MOE = (crit. val)(std. dev).
Divide each side by your critical value to cancel it out. Square each side to get rid of the radical. Cross multiply to get the n on the left of the equation so that it will be by itself, and divide the standard deviation to get your answer.
No comments:
Post a Comment