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Article

Effects of Quantitative Ordinal Scale Design on the Accuracy of Estimates of Mean Disease Severity

1
Division of Biometrics, Department of Agronomy, National Chung Hsing University, Taichung 40227, Taiwan
2
Department of Statistics and Information Science, Fu Jen Catholic University, New Taipei City 24205, Taiwan
3
Department of Plant Pathology, National Chung Hsing University, Taichung 40227, Taiwan
4
USDA-ARS-SEFTNRL, 21 Dunbar Road, Byron, GA 31008, USA
5
Graduate Institute of Food Safety, National Chung Hsing University, Taichung 40227, Taiwan
*
Author to whom correspondence should be addressed.
These authors contributed equally to this work.
Agronomy 2019, 9(9), 565; https://doi.org/10.3390/agronomy9090565
Received: 14 August 2019 / Revised: 8 September 2019 / Accepted: 17 September 2019 / Published: 19 September 2019
Estimates of plant disease severity are crucial to various practical and research-related needs in agriculture. Ordinal scales are used for categorizing severity into ordered classes. Certain characteristics of quantitative ordinal scale design may affect the accuracy of the specimen estimates and, consequently, affect the accuracy of the resulting mean disease severity for the sample. The aim of this study was to compare mean estimates based on various quantitative ordinal scale designs to the nearest percent estimates, and to investigate the effect of the number of classes in an ordinal scale on the accuracy of that mean. A simulation method was employed. The criterion for comparison was the mean squared error of the mean disease severity for each of the different scale designs used. The results indicate that scales with seven or more classes are preferable when actual mean disease severities of 50% or less are involved. Moreover, use of an amended 10% quantitative ordinal scale with additional classes at low severities resulted in a more accurate mean severity compared to most other scale designs at most mean disease severities. To further verify the simulation results, estimates of mean severity of pear scab on samples of leaves from orchards in Taiwan demonstrated similar results. These observations contribute to the development of plant disease assessment scales to improve the accuracy of estimates of mean disease severities. View Full-Text
Keywords: disease severity; disease scales; nearest percent estimates; mean squared error; plant epidemiology disease severity; disease scales; nearest percent estimates; mean squared error; plant epidemiology
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MDPI and ACS Style

Liu, H.I.; Tsai, J.R.; Chung, W.H.; Bock, C.H.; Chiang, K.S. Effects of Quantitative Ordinal Scale Design on the Accuracy of Estimates of Mean Disease Severity. Agronomy 2019, 9, 565. https://doi.org/10.3390/agronomy9090565

AMA Style

Liu HI, Tsai JR, Chung WH, Bock CH, Chiang KS. Effects of Quantitative Ordinal Scale Design on the Accuracy of Estimates of Mean Disease Severity. Agronomy. 2019; 9(9):565. https://doi.org/10.3390/agronomy9090565

Chicago/Turabian Style

Liu, Hung I., Jia R. Tsai, Wen H. Chung, Clive H. Bock, and Kuo S. Chiang. 2019. "Effects of Quantitative Ordinal Scale Design on the Accuracy of Estimates of Mean Disease Severity" Agronomy 9, no. 9: 565. https://doi.org/10.3390/agronomy9090565

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