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Table 2 The MRC grades 0-5 represent ordinal data

From: Using 4+ to grade near-normal muscle strength does not improve agreement

The ordinal data type is a sub-group of the categorical data type, in which different categories are ordered in relation to each other. An example of ordinal data could be a measure of experience, on a scale with three ordered categories: a) novice, b) experienced and c) expert. The MRC-scale is also an ordinal data type and the grade ‘4’ for instance, represents a muscle strength greater than ‘3’ and less than ‘5’. Confusingly, the difference in numerical value is 1 in both directions, but the step up from ‘3’ to ‘4’ is not necessarily the same as the step up from ‘4’ to ‘5’ in the same way that the difference between ‘novice’ and ‘experienced’ is not necessarily the same as that between ‘experienced’ and ‘expert’. Neither does a grade ‘2’ on the MRC-scale necessarily represent a meaningful mean value of ‘1’ and ‘3’. As such, the numbers only represent designations or names of categories on an ordinal scale and arguably it would have been less confusing to designate the steps on the scale in some other way, e.g. using letters A to F.