The Horsfall-Barratt scale is a system used in plant pathology to assess plant diseases where each plant is assigned a numerical value according to the percentage of leaf area showing disease symptoms. The Horsfall-Barratt scale was designed to compensate for human error in estimating the amount of disease present. The scale is as follows:
The scale is often used in modified form with index values from 0–11 or with different numbers of index points.
References
Horsfall-Barratt scale Wikipedia(Text) CC BY-SA