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Difficulty: MediumMeasures of Association and Impact

A randomized, double-blind, placebo-controlled trial evaluates the efficacy of a novel SGLT2 inhibitor for preventing heart failure hospitalizations in patients with type 2 diabetes. The study follows patients over a 5-year period. The results show that 200 out of 2,000 patients in the SGLT2 inhibitor group required hospitalization for heart failure. In the placebo group, 300 out of 2,000 patients required hospitalization for heart failure. Based on these trial findings, match each descriptive epidemiological concept to its correctly calculated numerical value.

  • The absolute difference in the risk of heart failure hospitalization between the treatment and placebo groups (Absolute Risk Reduction)5%
  • The proportion of baseline risk that is removed by the active therapy (Relative Risk Reduction)33%
  • The risk of the primary outcome in the treatment group compared to the placebo group (Relative Risk)0.67
  • The number of patients who must receive the SGLT2 inhibitor to prevent one additional heart failure hospitalization (Number Needed to Treat)20

Answer

The Absolute Risk Reduction matches 5%, the Relative Risk Reduction matches 33%, the Relative Risk matches 0.67, and the Number Needed to Treat matches 20.
Each epidemiological measure is derived directly from the baseline event rates. The placebo event rate (CER) is 15%15\%, and the treatment event rate (EER) is 10%10\%. ARR is the simple difference between these rates (15%−10%=5%15\% - 10\% = 5\%). NNT is the mathematical inverse of the ARR expressed as a decimal (1/0.05=201 / 0.05 = 20). RR is the direct ratio of the treatment risk to the placebo risk (10%/15%=0.6710\% / 15\% = 0.67). RRR expresses the ARR as a fraction of the original placebo risk (5%/15%=33%5\% / 15\% = 33\%).

Step-by-Step Solution

1
Calculate the Experimental Event Rate (EER) and Control Event Rate (CER).
EER = 200/2000=0.10200 / 2000 = 0.10 (10%10\%). CER = 300/2000=0.15300 / 2000 = 0.15 (15%15\%).
Baseline event rates are required to compute all measures of association and impact.
2
Calculate the Absolute Risk Reduction (ARR).
ARR=CER−EER=0.15−0.10=0.05ARR = CER - EER = 0.15 - 0.10 = 0.05 (or 5%5\%).
ARR represents the absolute difference in risk between the placebo and treatment groups.
3
Calculate the Relative Risk Reduction (RRR).
RRR=ARRCER=0.050.15=0.333RRR = \frac{ARR}{CER} = \frac{0.05}{0.15} = 0.333 (or 33%33\%).
RRR represents the proportion of the baseline risk that was eliminated by the treatment.
4
Calculate the Relative Risk (RR).
RR=EERCER=0.100.15=0.667RR = \frac{EER}{CER} = \frac{0.10}{0.15} = 0.667 (or 0.670.67).
RR compares the probability of the outcome occurring in the treatment group relative to the placebo group.
5
Calculate the Number Needed to Treat (NNT).
NNT=1ARR=10.05=20NNT = \frac{1}{ARR} = \frac{1}{0.05} = 20.
NNT quantifies the clinical impact, indicating how many patients must be treated to prevent one adverse event.

Key Concept

Measures of Association and Impact
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Topics

Measures of Association and Impact

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