--- /dev/null
+Reference
+<a href="https://en.wikipedia.org/wiki/Serial_interval" target="_blank"
+ ><i>Wikipedia</i></a
+>
+<br />
+<br />
+
+<p>The serial interval (SI) is NOT one of the estimators.</p>
+<p>
+ The SI is a parameter required by all of the estimators, and can also be
+ estimated by the WP method.
+</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 and their
+ subsequent transmissions.
+</p>