Average Response Time is calculated as an average of durations of the web transactions simulated on the target website at a given time interval:

Av.Response Time = ∑  time of transaction duration / the number of transactions started

Where a transaction is a sequence of completed operations performed on a web resource by a visitor or a sequence of HTTP(S) requests and replies. Time of transaction duration is a time elapsed from the moment the transaction is started at to the moment it is completed.

Example of a transaction: a sequence of such operations as loading a web page, logging in to the site, navigating to another page and submitting a web form. 

A transaction duration time DO NOT include a browser launch and shut down time.