Question 720:

Asked on May 5, 2009

Tags: Sample Size

1

Answer:

No answer provided yet.We need to work backwards from computing a confidence interval to find the correct sample size. One problem with this question is that we need to know what the acceptable margin of error is. We've been told that the director wants to be 95% confident in her estimate and this refers to the confidence level. However, we aren't given what margin of error around the years are acceptable, so I'm going to take a guess here. See if this question might have an additional part that was left off, or inform the instructor that part of the necessary information is missing.

I'm going to use 5% of the mean or .6 years as the acceptable margin around the mean (you can change it and update the equation if necessary).

Use the following Steps.

1. Divide the desired margin of error by the critical z-value for a 95% confidence level =.6/1.959964 = .306128
   
2. Divide the standard deviation by the result from Step 1 = 2/.306128 = 6.533213
3. Square the results from Step 2 = 42.68288.

Now we round this up to the nearest person to get 43. So the director would need to plan on sampling 43 subscribers to be 95% confident of the average age to within +/- .6 years.