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+<p>Reference: <a href="https://en.wikipedia.org/wiki/Serial_interval" target="_blank"><em>Wikipedia</em></a>
+<p>
+ The serial interval (SI) is not one of the estimators. It is a parameter required by most of the estimators, and can
+ also be estimated by some of them (if not specified).
+</p>
+<p>
+ The SI is defined as the average time between successive infections in a chain of transmission (i.e., the time between
+ the infection of an infected individual and their subsequent transmissions).
+</p>