With OmniFocus running, will log how many tasks in each context were marked completed in the last 7 days.

completed tasks.applescript

(*
  This script logs the number of tasks completed for each context in the last 7 days.
	
	Copyright © 2013, James Badger.
*)

property daysLimit : 7

on run
	tell application "OmniFocus"
		tell front document
			set allContexts to every context
			
			repeat with aContext in allContexts
				log name of aContext as string
				
				set complete_count to 0
				set theTasks to tasks of aContext
				repeat with aTask in theTasks
					if aTask is completed and my differenceInDays(current date, completion date of aTask) is less than daysLimit then
						set complete_count to complete_count + 1
					end if
				end repeat
				
				log complete_count
			end repeat
		end tell
	end tell
end run

on differenceInDays(first_date, second_date)
	return (first_date - second_date) / (3600 * 24)
end differenceInDays

sample output

(*Errands*)
(*2*)
(*Home*)
(*16*)
(*Waiting*)
(*0*)
(*Work*)
(*5*)