A dodgy sentence

[oortseam]

Hello there, I need some help as I'm translating some material into my native language and there's a certain sentence that sounds a bit dodgy. Here it goes:

(youtube /watch?v=9IRv3TOjE1I#t=10m05s)

Throughout last year, we've been - depending on which intervals you take - we've been consistently increasing case detection rates in five hospitals in Dar es Salaam between 30 and 40 percent.

I suppose he meant to say that they've been increasing case detection rates by between 30 and 40 percent; however, what does 'depending on which intervals you take' mean? I probably could guess, but I can't really base my translation on guesses.

Thanks in advance for your input!

[SoothingDave]

Deciding on exactly which dates you select to compute the rates, the calculated average could vary from 30-40 %.