Situation: The quality committee of a 200-bed hospital notes that the adult medical ward's rate of hospital-acquired pressure injury has risen for three quarters. The head nurse forms a team of staff nurses, a nursing attendant, a dietitian, and a physical therapist, which will work through the Plan-Do-Study-Act (PDSA) cycle. In the quarter before the change, the ward recorded 6 hospital-acquired pressure injuries in 2,400 patient-days. In the quarter after the change, it recorded 3 in 2,000 patient-days. By what percentage did the rate per 1,000 patient-days fall? Round off to the nearest whole number.

Rate before = (6 ÷ 2,400) × 1,000 = 2.5 per 1,000 patient-days; rate after = (3 ÷ 2,000) × 1,000 = 1.5 per 1,000 patient-days. The fall is 2.5 − 1.5 = 1.0, and 1.0 ÷ 2.5 × 100 = 40%. Counts cannot be compared directly because the number of patient-days differs between quarters.

Step 1. Calculate the rate before the change

The ward recorded 6 hospital-acquired pressure injuries in 2,400 patient-days. Because the denominator differs between quarters, the count alone is not comparable. The rate per 1,000 patient-days is calculated as:

(6 ÷ 2,400) × 1,000 = 2.5 per 1,000 patient-days.



Step 2. Calculate the rate after the change

After the intervention, the ward recorded 3 injuries in 2,000 patient-days:

(3 ÷ 2,000) × 1,000 = 1.5 per 1,000 patient-days.



Step 3. Compute the percentage reduction

The absolute fall in the rate is 2.5 − 1.5 = 1.0 per 1,000 patient-days. The percentage fall is:

(1.0 ÷ 2.5) × 100 = 40%.



Key point! When patient-days change between measurement periods, raw counts cannot be compared directly; the rate per 1,000 patient-days must be calculated first. This is the same logic used in hospital-acquired pressure injury surveillance, where incidence is expressed as a rate rather than a simple count ^[1][4].



Why this matters in pressure injury quality improvement

Hospital-acquired pressure injuries are a common patient safety indicator, and tracking them over time requires a stable denominator. In a 200-bed hospital, quarterly patient-days fluctuate with occupancy and length of stay. If the team had compared only the counts—6 before and 3 after—they might have concluded a 50% reduction, which would overstate the improvement because the after-period had fewer patient-days at risk. The correct comparison uses the incidence rate per 1,000 patient-days, yielding a 40% fall ^[1][4].



How this fits the PDSA cycle

The Plan-Do-Study-Act framework requires measurement before and after a change to judge whether the change led to improvement. In this scenario, the team would compare the pre-intervention rate (2.5 per 1,000 patient-days) with the post-intervention rate (1.5 per 1,000 patient-days). A 40% reduction suggests the intervention may be effective, but the team would also consider whether the change is sustained over additional PDSA cycles and whether other factors—such as changes in patient acuity or documentation practices—could explain the difference .



Watch out! A common error is to divide the count difference by the after-count or to compare raw counts directly. The percentage change must always be calculated relative to the baseline rate, not the post-intervention rate. Using the after-rate as the denominator would give (1.0 ÷ 1.5) × 100 = 67%, which is incorrect because the baseline rate is the reference point for measuring change.