What is a cost-effectiveness plane and how is it interpreted?

• A. Four-quadrant plot of incremental cost vs incremental effect; quadrants indicate more/less costly and more/less effective
• B. A chart of costs over time
• C. A diagram of patient flow
• D. A scatter plot of costs per patient

Answer: (D)

The cost-effectiveness plane is a four-quadrant graph used to visualize the trade‑off between cost and effect when comparing two health interventions. The horizontal axis shows the difference in effectiveness (incremental effect, such as QALYs gained), and the vertical axis shows the difference in cost (incremental cost). Each point represents an estimate of those differences (often from bootstrapped samples).

Interpretation comes from the quadrants:

• Top right: more expensive and more effective. You weigh the additional benefit against the extra cost.

• Bottom right: less costly and more effective. This is the desirable, “dominant” scenario.

• Top left: more expensive and less effective. This is unfavorable.

• Bottom left: less costly and less effective. It’s a trade-off where you save money but lose some effectiveness.

Decision making often involves a willingness-to-pay threshold for an extra unit of effect. By plotting many samples on the plane, you can see the probability the intervention is cost-effective at that threshold, or identify how often it falls in the dominant or unfavorable regions.

A scatter plot of costs per patient would not capture the essential comparison of incremental cost to incremental effect, so it wouldn’t convey the same interpretation as the cost-effectiveness plane.