Sample Size for Tolerance Intervals

Calculating Margin of Error - Normal Method

  

Increasing your sample size decreases your margin of error and thus makes your tolerance interval shorter. If you specify one or more sample sizes, Minitab calculates the margins of error that you can achieve with those sample sizes.

Example Output

Method

 

Confidence level                    95%

Percent of population in interval   95%

Margin of error probability        0.05

 

Margin of Error for 95% Tolerance Interval

 

Sample   Normal  Nonparametric    Achieved  Achieved Error

  Size   Method         Method  Confidence     Probability

    50  4.4015%        4.2846%       72.1%           0.050

   100  3.6914%        4.6435%       96.3%           0.050

 

Achieved confidence and achieved error probability apply only to nonparametric method.

Interpretation

The results show that, with 50 observations, the margin of error for a tolerance interval that is calculated using the normal method is 4.4015%. In other words, with 50 observations, there is only a 5% chance that the tolerance interval will contain 99.4015% or more of the population.

Increasing the sample size to 100 reduces the margin of error for the normal method to 3.6914%.