Details

Autor(en) / Beteiligte

• Chiang, K.S.
• Liu, H.I.
• Tsai, J.W.
• Tsai, J.R.
• Bock, C.H.

Titel

A discussion on disease severity index values. Part II: using the disease severity index for null hypothesis testing

Ist Teil von

• Annals of applied biology, 2017-11, Vol.171 (3), p.490-505

Ort / Verlag

Oxford, UK: Blackwell Publishing Ltd

Erscheinungsjahr

2017

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• A disease severity index (DSI) is a single number for summarising a large amount of information on disease severity. The DSI has most often been used with data based on a special type of ordinal scale comprising a series of consecutive ranges of defined numeric intervals, generally based on the percent area of symptoms presenting on the specimen(s). Plant pathologists and other professionals use such ordinal scale data in conjunction with a DSI (%) for treatment comparisons. The objective of this work is to explore the effects on both of different scales (i.e. those having equal or unequal classes, or different widths of intervals) and of the selection of values for scale intervals (i.e. the ordinal grade for the category or the midpoint value of the interval) on the null hypothesis test for the treatment comparison. A two‐stage simulation approach was employed to approximate the real mechanisms governing the disease‐severity sampling design. Subsequently, a meta‐analysis was performed to compare the effects of two treatments, which demonstrated that using quantitative ordinal rating grades or the midpoint conversion for the ranges of disease severity yielded very comparable results with respect to the power of hypothesis testing. However, the principal factor determining the power of the hypothesis test is the nature of the intervals, not the selection of values for ordinal scale intervals (i.e. not the mid‐point or ordinal grade). Although using the percent scale is always preferable, the results of this study provide a framework for developing improved research methods where the use of ordinal scales in conjunction with a DSI is either preferred or a necessity for comparing disease severities. Overview of the options for assessing disease severity, and specifically the process used for establishing a quantitative ordinal scale using a DSI regarding the number of grades, the nature of the grade values and the method (interval midpoint or grade values) for estimating mean disease severity and/or hypothesis testing. The results of this study demonstrated that an amended 10% category scale (10% linear scale emphasizing severities ≤ 50% disease, and with additional intervals at severities < 10%) provided both accuracy for estimating the disease severity, and optimal power for hypothesis testing. The boxes with the parallel grey bars indicate other methods for assessing disease severity, depending on purpose, desired accuracy and disease characteristics. Solid dark grey boxes indicate the process of DSI scale selection for estimating only the disease severity, while the pale gray boxes indicate the process of DSI selection for hypothesis testing.