What is the Incremental Cost-Effectiveness Ratio (ICER) and how is it calculated?

• A. ICER = (Cost B − Cost A) / (Effect B − Effect A)
• B. ICER = Cost A / Cost B
• C. ICER = (Cost A + Cost B) / (Effect A + Effect B)
• D. ICER = (Cost A − Cost B) / (Effect A − Effect B)

Answer: (D)

The main idea is to see how much extra you pay for each extra unit of effect when moving from one option to another. The Incremental Cost-Effectiveness Ratio is calculated as the difference in costs divided by the difference in effects between the two options: ICER = (Cost of the new option − Cost of the comparator) / (Effect of the new option − Effect of the comparator). If you swap the labels for the two options, both differences change sign, so the ratio stays the same.

For example, if the new option costs 120 and yields 0.8, while the comparator costs 100 and yields 0.6, the ICER is (120 − 100) / (0.8 − 0.6) = 20 / 0.2 = 100 per unit of effect. If you compute with the reversed form, (100 − 120) / (0.6 − 0.8) = (−20) / (−0.2) = 100, same result.

So the essence is the incremental cost over the incremental effect; the presented form is algebraically equivalent when the labels are applied consistently.