CPHQ Statistical Process Control 5 — Questions and Answers
Question 1: A neonatal ICU tracks central-line-associated bloodstream infections (CLABSIs) per 1,000 line-days. The monthly line-days vary. Which chart is most appropriate?
- np-chart
- c-chart
- u-chart (Correct answer)
- p-chart
Correct answer: u-chart
The u-chart is ideal for rare event counts (CLABSIs) per variable unit of exposure (line-days), adjusting control limits for varying denominators.
Question 2: A Shewhart control chart uses 3-sigma limits. What probability of a false alarm does this correspond to for normally distributed data?
- 5.0%
- 2.5%
- 0.27% (Correct answer)
- 1.0%
Correct answer: 0.27%
For normally distributed data, 99.73% of points fall within ±3σ, leaving a 0.27% false-alarm probability per point.
Question 3: When should a healthcare team recalculate control chart limits?
- Every month regardless of process changes
- After a confirmed, sustained process change or improvement (Correct answer)
- Whenever a point falls outside control limits
- When the sample size changes by one unit
Correct answer: After a confirmed, sustained process change or improvement
Control limits should be recalculated when a verified, sustained process change has occurred—not in response to individual signals or routine schedules.
Question 4: A quality analyst is choosing between an X-bar & R chart and an X-bar & S chart for monitoring blood glucose turnaround times with subgroup size n=12. Which should she choose and why?
- X-bar & R, because ranges are always more accurate for large samples
- X-bar & S, because the S chart is more efficient for subgroup sizes larger than about 8–10 (Correct answer)
- X-bar & R, because ranges are easier to compute than standard deviations
- X-bar & S, because S charts work only for subgroup sizes above 10
Correct answer: X-bar & S, because the S chart is more efficient for subgroup sizes larger than about 8–10
For subgroups larger than 8–10, the S chart (using standard deviation) is statistically more efficient than the R chart (using range) at detecting variability changes.
Question 5: A hospital's patient fall rate has been stable for 18 months. Administration mandates a new rounding protocol. After implementation, five consecutive months show rates below the previous centerline. What is the appropriate SPC action?
- Widen control limits to include the new lower values
- Declare a special cause signal and investigate the improvement (Correct answer)
- Continue monitoring without any chart modification
- Remove the five outlier months from the baseline
Correct answer: Declare a special cause signal and investigate the improvement
Five consecutive points below the centerline violate a run rule, signaling a special cause—in this case, likely a beneficial process change worth investigating and institutionalizing.
Question 6: In SPC terminology, a process is said to be 'in statistical control' when:
- All outputs meet customer specification limits
- Only common cause variation is present and no control chart rules are violated (Correct answer)
- The Cpk value exceeds 1.33
- The process average equals the target value
Correct answer: Only common cause variation is present and no control chart rules are violated
Statistical control means the process exhibits only random (common cause) variation with no signals of special causes on the control chart.
Question 7: Which action is appropriate when special cause variation is identified on a healthcare control chart?
- Adjust the process mean to the centerline immediately
- Identify and eliminate the root cause of the special cause (Correct answer)
- Increase sample size to reduce false alarms
- Recalculate control limits using tighter sigma values
Correct answer: Identify and eliminate the root cause of the special cause
Special cause variation requires root cause analysis and elimination; attempting to adjust a process for special causes without investigation leads to over-control (tampering).
CPHQ Statistical Process Control 5
A neonatal ICU tracks central-line-associated bloodstream infections (CLABSIs) per 1,000 line-days.
The monthly line-days vary.
Which chart is most appropriate?