Which of these statements describe an undesirable situation when implementing SPC? (Note: There are 2 correct answers).

Question

Options

• AThe lower Control Limit for the R chart is equal to zero
• BThe Control Limits are wider than the customer specification limits
• CA process is in Statistical Control before implementation of SPC
• DAttempt to use SPC for tracking transaction times at a warehouse
• EIndication of the specification limits on the Control Chart

• A
  3% (1)
• B
  78% (29)
• C
  5% (2)
• D
  14% (5)

Explanation

B is undesirable because when control limits are wider than specification limits, the process is statistically incapable - it will produce defects even while appearing "in control." This means the process naturally varies beyond what the customer will accept, and SPC alone cannot fix an incapable process. E is undesirable because plotting specification limits directly on a control chart conflates two fundamentally different concepts: control limits reflect natural process variation (±3σ), while spec limits reflect customer requirements - mixing them encourages operators to tamper with a stable process or misjudge capability.

Why the distractors are wrong:

• A is normal - an R-chart LCL of zero is mathematically expected for small sample sizes (n ≤ 6, where D3 = 0), not a problem.
• C is actually desirable - you want a stable, in-control process as your baseline before implementing SPC so control limits are meaningful.
• D is acceptable - SPC works perfectly well for transactional/service metrics like time-based data; it's not limited to manufacturing.

Memory tip: Remember "two separate worlds" - the Control Chart world (process stability) and the Specification world (customer requirements). Any situation that merges these worlds is undesirable: either the process bleeds into spec territory (B) or specs bleed onto the chart (E).

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