Enter your data in the blue cells and the answers will be in the yellow cells.

# confidence interval

## Minimum Sample Size for Population Mean given c, sigma, E

Enter your data in the blue cells and the minimum sample size will be in the yellow cell at the bottom. Note: the standard deviation and E must be in the same units (e.g. inches, feet, etc).

Download a copy of the workbook Min Sample Size for Mean CI V1.00

## Confidence Interval for a Mean using StatCrunch

If you are given summary data and asked to find a confidence interval, it is relatively easy using StatCrunch. Here is a typical problem:

In this problem, you are told to use the t-distribution, but that may not always be the case. If you are given the sample standard deviation, s, instead of the population standard deviation, σ , use the t-distribution to find the confidence interval. In StatCrunch, this is in the T Stats menu. If you are given σ or are told to use the z-distribution, it is essentially the same steps except you select Z Stats in step 2.

Download a PDF for your notes: Confidence Interval for a Mean using StatCrunch

## Minimum Sample Size Calculator for Population Proportion Confidence Interval

If you are conducting a survey, you will need to decide on the number of people you need to sample. That, of course, depends upon how confident you want to be and how accurately you want to predict the population proportion based on your survey results. If you want to be 95% confident, that is your level of confidence, C. If you want the population proportion within + or – 2%, that is the margin of error, E, you will accept. With that information, you can find the minimum number of people to be surveyed.

If you have an estimate of the population proportion, use that as p-hat. If you do not have any prior information, use 0.5 as your initial estimate of p-hat as that will give you the largest minimum sample size.

You can do that using StatCrunch (here), but I find it quicker to use my Excel calculator below. Just enter your data in the blue cells and the answer will be in the yellow cell.

Here is the answer to part a, with no prior estimate of p-hat. The sample size is 752.

If you use the estimate of 0.34 instead of 0.5, the minimum sample size decreases to 675. The sample size decreased, which is what you should expect. Using 0.5 as the estimate when you don’t have other information will give the largest sample size.

Download a copy of the workbook here: Minimum Sample Size for Proportion CI V1.00

## Find Confidence Level, C, for Population Proportion Confidence Interval

Problem 6.3.27 gives you some summary data about a survey and asks you to find the confidence interval for the population proportion if the margin of error, E, is also given. Although you can do this problem using StatCrunch, I find it to be one of those problems where Excel is quicker and easier, particularly if you use my “calculator.” Enter your summary data and the E. Find the answers in the yellow cells below. You may also download a copy of this workbook.

Download a copy Confidence Level for Proportion CI V1.00