Question 151:

Asked on April 14, 2008

Tags: Confidence Intervals , t-statistic , t-distribution , TINV , Margin of Error , Standard Error of the Mean

1

Answer:

No answer provided yet.

Since this sample size is relatively small (25) I recommend building your confidence interval using the a t-statistic. Assuming the data is roughly normally distributed:

1. Calculate the standard error of the mean: standard deviation / square root of the sample size = .35/5 = .07
2. Find the critical value from the t-distribution for 24 degrees of freedom and a probability of .05. You can use MS Excel and type =TINV(.05,24) and should get 2.06.
3. Calculate the margin of error: which is the standard error of the mean times the t-value = .07*2.06 = .1442.
4. Generate the Lower and Upper bounds of the confidence interval by adding and subtracting the margin of error to the mean= 2.3+.1442 = 2.442 and = 2.3-.1442 =2.1558

So your 95% confidence interval around the mean is (2.1558, 2.442).