Home > What is SPC?

- Control Charts for Attribute Data
- p-charts for proportion nonconforming units, sample size not necessarily constant
- ¬ When to use a p-chart
- • When it is difficult or uneconomical to make a numerical measurement
- • When it is desired to combine different types of defects into an overall proportion
- • When the available data are for attributes
- • When the data come from a binomial process
- ¬ Management summaries
- • Many management summaries are attribute forms and could benefit from control chart analysis.
- • Examples: scrap rates, quality audits, first-run yields, etc.
- np-charts for number of nonconforming units, sample size is constant
- ¬ When to use an np-chart?
- • Use the same criteria as a p-chart:
- (a) When it is difficult or uneconomical to make a numerical measurement
- (b) When it is desired to combine different types of defectives into a single value
- (c) When the available data are for attributes
- (d) When the data come from a binomial process
- • And the subgroups are all the same size
- ¬ How is it different from a p-chart?
- • The actual number of nonconforming parts are plotted, rather than the proportion defective
- u-chart for the number of defects per inspection unit, sample size not necessarily constant
- c-chart for the number of defects, sample size constant
- ¬ When to use c- or u-charts
- • When the data is attribute data of defects, not defectives
- • When the data comes from a Poisson process
- • c-charts are charts constructed for number of occurrences for a constant exposure
- • u-charts are constructed for rate of occurrence for either constant or varying exposure
- ¬ Examples
- • Each month, 100 invoices are audited and the total number of mistakes is recorded.
- • In a molding process, there is a problem with pinholes in plastic bottles. Each day, a number of bottles are examined and the number of pinholes are recorded.<
- • Each month, the number of accidents that occur in an organization is recorded.
- Control Charts for Variable Data
- X-R Chart
- ¬ The X-R chart is the most versatile of control charts, and is used in most applications.
- ¬ Charting of averages and charting of ranges are used to check if a constant-cause system exists.
- • X-chart measures variability between samples
- • R-chart measures variability within samples
- • For sample size n > 10, R loses its efficiency in estimating process sigma and R-chart may not be appropriate.
- X-S Chart
- ¬ The S Chart may be used when n is not constant
- ¬ For large sample size (n 10), the range loses its efficiency as an estimator of
- The I-MR Chart is a useful control chart if the characteristic is independently and normally distributed. Applications where sample size for process monitoring is n=1.
- ¬ Measurement is expensive, e.g. destructive testing
- ¬ Production rate is very slow