Bland-Altman comparison of two methods for assessing severity of Verticillium wilt of potato

Abstract

The agreement between two disease assessment approaches is important to know prior to replacing or interchanging the use of an established method with a recently developed method of measurement. Frequently used statistical methods to compare two different disease rating methods is the Pearson correlation coefficient or the ordinary least square regression (OLS), but they have their shortcomings. Bland-Altman proposed an alternative method for studying agreement between methods using simple graphs and basic statistics. Traditionally, when disease management strategies are being evaluated in the field, the severity of the disease is estimated using a visual assessment. Canopeo, designed by the Oklahoma State University app center, is a smart phone app designed for measuring green canopy cover. Thus, the aim of this study was to explain the Bland-Altman method with examples of visual and Canopeo methods of wilt measurement. Symptoms of Verticillium wilt in potato were estimated (repeated measures) in two trials using Canopeo and a traditional visual assessment method. Complete wilt data (repeated measures) were considered for studying the agreement between visual and Canopeo assessments. A preset cutoff limit of ≤5% bias (total allowable) between rating methods was considered acceptable prior to using the Bland-Altman comparison. The Bland-Altman method for determining the agreement in wilt severity methods in trial 1 and trial 2 estimated that the mean difference between rating methods were 5.10 and 5.91%, respectively. A mean difference greater than five indicates that the methods of measuring wilt are not in agreement. The study reported here demonstrates that Pearson correlation and OLS regression are inappropriate for assessing the agreement between two methods of measurement.