Data considerations for Between/Within Capability Sixpack

To ensure that your results are valid, consider the following guidelines when you collect data, perform the analysis, and interpret your results.

The data should be continuous

Continuous data are measurements that may potentially take on any numeric value within a range of values along a continuous scale, including fractional or decimal values. Common examples include measurements such as length, weight, and temperature.

If you have attribute data, such as counts of defectives or defects, use Binomial Capability Analysis or Poisson Capability Analysis.

Collect enough data to obtain reliable estimates of process capability
Try to collect at least 100 total data points (subgroup size*number of subgroups), such as 25 subgroups of size 4, or 35 subgroups of size 3. If you do not collect a sufficient amount of data over a long enough period of time, the data may not accurately represent different sources of process variation and the estimates may not indicate the true capability of your process.
Data should be collected in rational subgroups, if possible
A rational subgroup is a small sample of similar items that are produced over a short period of time and that are representative of the process. Observations for each subgroup should be collected under the same inputs and conditions, such as personnel, environment, or equipment. If you do not collect rational subgroups, the variation in the subgroups may reflect special causes rather than the natural, inherent variation of the process.
All subgroups should be the same size, if possible
If your subgroups are not all the same size, because of missing data or unequal sample sizes, then Minitab uses only subgroups of the most common size to estimate the between-subgroup variation.
Each subgroup should have two or more observations
If you do not have two or more observations in at least one subgroup, Minitab cannot perform the analysis because between-subgroup variation cannot be estimated separately from within-subgroup variation.
The process must be stable and in control
If the current process is not stable, then the capability indices cannot be reliably used to assess the future, ongoing capability of the process. Use the control charts in the capability sixpack output to determine whether the process is stable and in control.
The data should follow a normal distribution
The process capability estimates for this analysis are based on the normal distribution. If the data are not normally distributed, the capability estimates will not be accurate. If your data are nonnormal, you can transform them using the Box-Cox transformation, which is included in the Box-Cox options of this analysis. Use the normal probability plot and the histogram in the capability sixpack output to determine whether the data (or the transformed data) follow a normal distribution.
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