Control Charts & Analysis Flashcards
7 cards from real SPC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Control Charts & Analysis flashcards as text
An EWMA chart applies exponential weights to historical data primarily to:
Answer: Increase sensitivity to small process shifts by incorporating past data
The EWMA (Exponentially Weighted Moving Average) chart weights recent observations more heavily and accumulates past information to detect small mean shifts faster than a Shewhart chart.
On an X-bar chart, if the control limits were calculated using only out-of-control data points, the resulting chart would likely have:
Answer: Limits that are too wide, masking real signals
Including out-of-control (special cause) data inflates the estimated sigma, producing artificially wide control limits that hide future real signals.
Which of the following best describes 'rational subgrouping'?
Answer: Forming subgroups to maximize variation between groups and minimize variation within groups
Rational subgrouping structures samples so that within-subgroup variation reflects only common causes, while between-subgroup variation captures process changes over time.
A control chart shows a sawtooth pattern repeating every 8 data points. The most likely cause is:
Answer: A cyclical special cause such as shift changes or tooling wear
Periodic cyclical patterns in control charts often correspond to systematic process inputs that repeat on a fixed schedule, such as shift changes, batch cycles, or maintenance intervals.
What is the effect on control chart sensitivity of increasing subgroup size from n=2 to n=9?
Answer: Control limits narrow, increasing sensitivity to mean shifts
Larger subgroup sizes reduce the standard error of the mean (sigma/√n), narrowing the control limits and making the chart more sensitive to smaller process shifts.
In a Laney p'-chart, the correction factor beta (β) is used to address which limitation of the standard p-chart?
Answer: Overdispersion caused by between-subgroup variation exceeding binomial expectations
The Laney p'-chart corrects for overdispersion — when between-subgroup variation exceeds what the binomial model predicts — by incorporating a Z-score smoothing factor β.
A process engineer observes that the R chart is in control but the X-bar chart shows multiple out-of-control points. The best interpretation is:
Answer: Process variability is stable but the process mean is shifting
An in-control R chart with an out-of-control X-bar chart indicates stable within-subgroup spread while the process average is experiencing special cause shifts over time.