Monthly central-line infection rates have fluctuated around 1.8 per 1,000 line-days for 18 months. After a sterile-insertion bundle, the next eight

Question

Monthly central-line infection rates have fluctuated around 1.8 per 1,000 line-days for 18 months. After a sterile-insertion bundle, the next eight monthly points all lie below the established center line, although none crosses a three-sigma control limit. What is the best interpretation?

Accepted Answer

Treat the eight-point run as special-cause improvement and investigate the associated process change

Answer

Treat the pattern as common-cause variation because no point crossed a control limit

Answer

Discard the post-intervention observations as regression to the mean because they remain within the limits

Answer

Treat the pattern as proof that the bundle alone caused the reduction and stop concurrent surveillance

Answer

Recalculate the center line and control limits immediately using the first post-intervention point

Root cause analysis — USMLE Step 3 MCQ

Monthly central-line infection rates have fluctuated around 1.8 per 1,000 line-days for 18 months. After a sterile-insertion bundle, the next eight monthly points all lie below the established center line, although none crosses a three-sigma control limit. What is the best interpretation?