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Text: The risk ratio does not have any dimensions or units attached to it, because it is a simple ratio and the units cancel out. However, there might be circumstances in which the RR varies over time, such that a long period of observation would provide a different estimate of RR than a much shorter observation time. Most people ignore this, since in the context of a given study, the observation period is usually explicitly stated, but ideally the interpretation of a risk ratio should mention the time period used to estimate the ratio.