* '''R''' (response time of the system): the response time depends on the system, the command and the context of the command. It is only used when the user actually has to wait for the system. For instance, when the user mentally prepares (M) for executing histheir next physical action only the non-overlapping part of the response time is needed for R because the user uses the response time for the M operation (e.g. R of 2 seconds – M of 1.35 seconds = R of .65 seconds). To make things clearer, Kieras <ref name=Kieras>{{cite web|last1=Kieras|first1=David|title=Using the Keystroke-Level Model to Estimate Execution Times|url=http://www-personal.umich.edu/~itm/688/KierasKLMTutorial2001.pdf|accessdate=22 June 2015|ref=9|page=3}}</ref> suggests the naming waiting time (W) instead of response time (R) to avoid confusion. Sauro suggests taking a sample of the system response time.<ref>{{cite book|last1=Sauro|first1=Jeff|title=Human-Computer Interaction. New Trends|editor1-last=Jacko|editor1-first=Julie A|chapter=Estimating productivity: Composite operators for keystroke level modeling|journal=Human-Computer Interaction. New Trends: Proceedings of the 13th International Conference (LNCS)|date=2009|volume=5610|page=355|doi=10.1007/978-3-642-02574-7_40|ref=10|publisher=Springer-Verlag|___location=Berlin Heidelberg|series=Lecture Notes in Computer Science|isbn=978-3-642-02573-0}}</ref>
The following table shows an overview of the times for the mentioned operators as well as the times for suggested operators:
