Figure 1

Variation between readers in asexual parasites counts from cross-sectional surveys of malaria in northeastern Tanzania[29]. The vertical axes show between-reader differences in terms of a) absolute difference in counts, b) ratio of counts and c) difference in square root counts. Different types of mean are used on the horizontal axes to ensure that the two axes are uncorrelated: arithmetic mean, geometric mean, and square mean root, respectively. Pairs with a zero reading (n = 702, or 45% of 1564) are not included in panel b) because they give rise to infinite ratios. To increase comparability, they also are omitted from the other two panels. Hence each panel shows 862 paired readings. In panel a), differences in either direction of more than 250 parasites (42 pairs, or 5% of 862) are plotted together at the ends of the vertical axis. This is to stop outliers dominating the figure. This is also done in panel c) for differences in square roots greater than 13 (16 pairs, or 2%). In panel c), the outer pair of horizontal lines shows the 95% range of the difference in square roots: -5.3 to 4.7. The inner pair of horizontal lines shows the range expected under the lower limit on this 95% range from the Poisson distribution: -1.39 to +1.39. In all three panels, the points are green circles or blue triangles, according to whether their difference in square roots is inside or outside this range.