Sample Size for Tolerance Intervals

Calculating Sample Size - Normal Method

  

Smaller margins of error require larger sample sizes. If you specify one or more values for the margin of error, Minitab calculates the sample sizes that are required to achieve those margins of error.

Example Output

Method

 

Confidence level                    95%

Percent of population in interval   95%

Margin of error probability        0.05

 

Sample size for 95% Tolerance Interval

 

  Margin  Normal  Nonparametric    Achieved  Achieved Error

of Error  Method         Method  Confidence     Probability

      1%    2480           4654       95.0%           0.049

      2%     525           1036       95.1%           0.048

 

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

Interpretation

The results of the sample size calculations show that, to achieve a margin of error of 1% using the normal method, you need to collect 2480 observations. In other words, with 2480 observations, the probability that a tolerance interval will contain 96% or more of the population is only 0.05.

If you are willing to accept a 2% margin of error, you need only 525 observations using the normal method.