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The Effect of Horsfall-Barratt Category Size on the Accuracy and Reliability of Estimates of Pecan Scab Severity

Pecan scab (Fusicladium effusum) is a destructive pecan disease. Disease assessments may be made using interval-scale-based methods or estimates of severity to the nearest percent area diseased. To explore the effects of rating method—Horsfall-Barratt (H-B) scale estimates versus nearest percent estimates (NPEs)—on the accuracy and reliability of severity estimates over different actual pecan scab severity ranges on fruit valves, raters assessed two cohorts of images with actual area (0 to 6, 6⁺ to 25%, and 25⁺ to 75%) diseased. Mean estimated disease within each actual disease severity range varied substantially. Means estimated by NPE within each actual disease severity range were not necessarily good predictors of the H-B scale estimate at <25% severity. H-B estimates by raters most often placed severity in the wrong category compared with actual disease. Measures of bias, accuracy, precision, and agreement using Lin's concordance correlation depended on the range of actual severity, with improvements increasing with actual disease severity category (from 0 to 6 through 25⁺ to 75%); however, the improvement was unaffected by the H-B assessments. Bootstrap analysis indicated that NPEs provided either equally good or more accurate and precise estimate of disease compared with the H-B scale at severities of 25⁺ to 75%. Inter-rater reliability using NPEs was greater at 25⁺ to 75% actual disease severity compared with using the H-B scale. Using NPEs compared with the H-B scale will more often result in more precise and accurate estimates of pecan scab severity, particularly when estimating actual disease severities of 25⁺ to 75%.