CPHQ Statistical Process Control 4 — Questions and Answers
Question 1: When plotting an Individuals (I) chart, the moving range is typically calculated using how many consecutive observations?
- 3 consecutive points
- 2 consecutive points (Correct answer)
- 5 consecutive points
- All points from the baseline period
Correct answer: 2 consecutive points
The moving range in an I-MR chart uses successive pairs of two consecutive observations to estimate short-term process variation.
Question 2: A CPHQ exam scenario describes a Sigma level of 4.0 for a medication dispensing process. Approximately what percentage of outputs fall outside specification limits?
- 6.68%
- 0.62% (Correct answer)
- 0.0063%
- 2.28%
Correct answer: 0.62%
A 4-sigma process has approximately 0.62% of outputs outside specification limits (6,210 DPMO), reflecting high but not Six Sigma quality.
Question 3: A quality team observes that all 15 consecutive points on a control chart fall within the 1σ zone of the centerline. Which pattern does this represent?
- Excellent process stability requiring no action
- Stratification, suggesting data from multiple process streams is mixed (Correct answer)
- A process shift toward the mean
- Normal common cause variation
Correct answer: Stratification, suggesting data from multiple process streams is mixed
Fifteen consecutive points within 1σ (Western Electric Rule 5) suggests stratification—data is likely sampled from different sub-populations with different means.
Question 4: For the CPHQ exam, which measure is used as the centerline of an R chart?
- Median of all ranges
- Average of subgroup ranges (R-bar) (Correct answer)
- Maximum range observed
- Process standard deviation × 2
Correct answer: Average of subgroup ranges (R-bar)
The centerline of an R chart is R-bar, the average of the subgroup ranges collected during the baseline period.
Question 5: A healthcare quality director wants to assess long-term process performance. Which capability metric should she use?
- Cp, based on within-subgroup variation
- Pp, based on total observed variation (Correct answer)
- Cpk, based on average subgroup range
- Sigma level derived from control limits
Correct answer: Pp, based on total observed variation
Pp (and Ppk) use total process variation (overall standard deviation), reflecting long-term performance, while Cp/Cpk use short-term within-subgroup variation.
Question 6: Which of the following is a correct interpretation of an out-of-control signal on a control chart?
- The process is producing defective output
- A special cause of variation is likely present and should be investigated (Correct answer)
- The process must be shut down immediately
- The specification limits need to be widened
Correct answer: A special cause of variation is likely present and should be investigated
An out-of-control signal means a special cause is likely present, triggering investigation—it does not necessarily mean defects are being produced.
Question 7: In rational subgrouping for an SPC chart in a hospital laboratory, samples should be grouped to:
- Maximize variation within subgroups and minimize variation between subgroups
- Minimize variation within subgroups and allow between-subgroup variation to represent process changes (Correct answer)
- Mix data from different shifts to increase sample size
- Ensure each subgroup spans the full production day
Correct answer: Minimize variation within subgroups and allow between-subgroup variation to represent process changes
Rational subgrouping minimizes within-subgroup variation (produced under similar conditions) so that between-subgroup differences reveal true process changes.
CPHQ Statistical Process Control 4
When plotting an Individuals (I) chart, the moving range is typically calculated using how many consecutive observations?