Ratio data: understanding pitfalls and knowing when to standardise

Ratios represent a single-value metric but consist of two component parts: a numerator variable and a denominator variable. Strictly speaking, a ratio is defined as: “the quantitative relation between two amounts showing the number of times one value contains or is contained by another”. When we discuss symmetry in sport science, we are generally comparing values of some metric between left and right sides or between agonist and antagonist muscles. The typical practice is to express the comparison as a ratio (differences are also a way of standardizing under different assumptions), such as the injured limb having only 60% of the strength of the uninjured limb. Conceptually though, we are using the ratio as one way to standardize the value of one variable with respect to another. Despite their common use, the interpretation of ratio standardisation, whether for symmetry or other reasons, often provides challenges, some of which are not always obvious to practitioners. Typically, when monitoring a change in ratios, if an intervention affects both the numerator and denominator, there will likely be challenges in interpreting the ratio appropriately. Therefore, the aim of this editorial is to use some examples to highlight when using this form of standardisation may be helpful, and when using it can lead to misinterpretations.

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