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 in time order
- Because run charts detect patterns over time, the order of the data is important. You should enter the data in the order it was collected, with the earliest data at the top of the worksheet.
- Data may be collected as individual observations or in subgroups
- A subgroup is a collection of similar items that are representative of the output from the process you want to evaluate. The items in each subgroup should be collected under the same process conditions, such as personnel, equipment, suppliers, or environment.
- Use individual observations when it is difficult or impossible to group measurements into subgroups. This situation frequently occurs when measurements are expensive, production volume is low, or products have a long cycle time.
- Collect enough data to represent the expected range of variation
- The data must include at least 3 data points. However, you should collect enough data to represent patterns over the time period of interest.