A graphical way of representing confidence intervals in cost-effectiveness analysis or cost-utility analysis. Confidence boxes show where a specified percentage (for example 95 per cent) of the data in a scatter plot will lie. Consider a health care technology that is costlier but also more effective than its comparator, so that we are in the north east quadrant of the cost-effectiveness plane shown in the figure. The slope of rays such as a and b shows the incremental cost-effectiveness ratio (AC/AE) of the technology under investigation relative to an alternative (control). The steeper the ray, the greater the marginal cost per marginal gain in output compared with the comparator. The upper and lower confidence limits of incremental cost are plotted against the upper and lower confidence limits of the effectiveness measure in the form of a box. Rays a and b are the outer limits of the confidence interval (usually 95 per cent). Cf. Confidence Ellipse.

Change in cost (AC)

Change in cost (AC)

Change in effectiveness (AE)

Confidence Ellipsoid 63

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