Scaling the indicator reaction with respect to τ = t/t s 2 is no accident. This is because, as mentioned previously, t s 2 gives a very good estimate of the completion timescale corresponding to the indicator reaction when the non-observable reaction is extremely fast in comparison. Moreover, the ratio δ S should give a good indication of how well the indicator reaction "keeps up" with the non-observable reaction. Since the completion of the indicator reaction cannot occur before the completion of the non-observable reaction, it stands to reason that if δ S 1, then the completion of the indicator reaction will occur at roughly the same time as the non-observable reaction .