23.Advanced Control [Charts]
X-bar & R Chart
- Data is variable (continuous)
- Data can be grouped into subgroups, letting you chart the mean of each group
o Production mean per hour
o Average answer speed of phones per hour
o Average number of customers each day
o Defects per hour
- Data count in each subgroup is less than 8 (although you can use the x-bar & R chart for subgroup sizes up to 100 in Minitab)
- Presents two charts
o The x-bar control chart plots the mean of each subgroup
o The R chart plots the range
X-bar & S Chart
- Data is variable (continuous)
- Data can be grouped into subgroups, letting you chart the mean of each group
- Data count in each subgroup is more than 8
- Sigma can be easily calculated (otherwise, use x-bar &R)
- Presents two charts
o The x-bar control chart plots the mean of each subgroup
o The S chart plots the standard deviation
I & MR Chart
- Data is variable (continuous)
- Data cannot be easily grouped into reasonable subgroups, so you must track individual data points instead of subgroup means
o Data is very difficult or expensive to obtain
o Production is very slow, so waiting for enough data to create subgroups would take too much time (for example, tracking data about surgeries performed in a small outpatient facility)
o Products have a low cycle time
P-Chart
- Data is discrete
- Data is presented as a percent (percent defective) rather than a count
- Doesn’t require a constant sample/subgroup size
- Plots the proportion of units that are nonconforming (are defective, don’t meet specifications)
- Use when:
o it is impossible, very time-consuming, or not financially feasible to measure or analyze numerical measurements
o sample or subgroup sizes are not equal, so the percent of defect is more important that the number of defects per group (as you can’t compare numbers in a smaller group with numbers in a larger group accurately)
o data is rate-based because it comes from a binomial or attribute process: the measurement or process is pass/fail, go/no go, etc. You can’t plot 1 or 0 on a control chart, but you can plot the percent of 1 or 0 in each sample.
NP-Chart
- Data is discrete
- Data is presented as a percent (percent defective) rather than a count
- Does require a constant sample size to be of use; other than this, you can use it for anything that you would use a p-chart for
- Plots the number of units that are nonconforming in each sample size
U-Chart
- Data is discrete
- Data is presented as a count (number of defects)
- Doesn’t require a constant sample/subgroup size
- Plots the number of defects per unit
- Use when data is about the defects themselves, not the overall defective product
C-Chart
- Data is discrete
- Data is presented as a count (number of defects)
- Does require a constant sample/subgroup size
- Plots the number of defects per sample
- Use when data is about the defects themselves, not the overall defective product