THE EFFECT OF USING A HORSFALL-BARRATT SCALE ON PRECISION AND ACCURACY OF VISUAL ESTIMATION OF POTATO LATE BLIGHT SEVERITY IN THE FIELD

Assessments of potato late blight severity by eleven people using dire ctly estimated percentage severity (EPS) of potato late blight caused by Phytophthora infestans were compared with the assessments of anothe r group of ten people using a Horsfall-Barratt scale with wider interv als in the mid range of severity (25-75% infection). This scale is fre quently used at the International Potato Center (CIP) to select for re sistant germplasm. EPS readings had greater variability among assessor s at mid range of severity than at extremes, as predicted by Horsfall and Barratt (1945). Transformation of EPS to CIP scale units or logits corrected uneven variances. Direct utilization of the CIP scale in th e field, however, did not correct uneven variances. This may have been due in part to subconscious linearization of scale intervals, which a re based on logarithms. Direct comparisons made by converting EPS to C IP scale levels demonstrated that precision and accuracy were greater among assessors using EPS.