Upper thresholds for response times ??


I've been asked to determine the lower and upper thresholds for our response times for the fire department for which I work. I watched this video: https://vimeo.com/145061136 (and I actually took a 3 day course last year given by the presenter of this video).

My question/problem isn't with calculating the thresholds the first time. It is: Do I ever need to recalculate the thresholds? If so, at what point, i.e. is it yearly or when policy changes and we see an improvement or worsening of the response times? Or, once they are set there isn't a need to update them.

I will be using response times for 1st on scene units, responding lights-and-sirens, for 2016 to set the initial thresholds. There will be 2 separate analysis: one for fire calls and one for EMS calls.

Any direction on this relatively minor dilemma would be greatly appreciated.

Thank you!