Background and Purpose: Analyses of trends in the delivery of time critical treatments typically report the median, but measures of central tendency may ignore important changes for specific patient groups. We considered whether this was an important effect during comparison of onset to treatment (OTT) time between 2 cohorts of patients with stroke receiving intravenous thrombolysis. Methods: After controlling for stroke severity, a relative distributions technique compared OTT for UK patients treated with recombinant tissue-type plasminogen activator registered within the first and last quarters (each n=661) of the Safe Implementation of Thrombolysis in Stroke-Monitoring Study database between January 2003 and September 2010. Results: Significant differences were found between OTT distributions. Overall, the second cohort’s OTT distribution demonstrated simultaneous increases in the proportion of patients with faster and slower OTT, which resulted in no net effect on the median after correction for stroke severity. Conclusions: Medians did not adequately describe distributional changes. Faster OTT may be because of more efficient processes in acute stroke centers and improved symptom recognition by the public. Slower OTT is likely to reflect movement from a 3- to a 4.5-hour OTT target. Relative distributions offer new insights into historical trends and service evaluation where time critical treatments are involved.