When I started 3 years ago with tactical exercises I didn't know about the importance of repetition.
I started with 5333+1 of papa Polgar.
Because of the apparent repetition there when you train the mate in one, I was inclined to think: well, if I do enough exercises the repetition comes by itself. But chess is such a rich game that the problems didn't have much overlap in patterns.
What have I done:
5333+1 Chess exercises from Laszlo Polgar
1200 exercises from a Dutch Course
500 exercises from Middlegames by papa Polgar
200 exercises from Endgames by papa Polgar
200 exercises from CD Chess Endgame Training by Convekta
4139 exercises from CD Intensive Course tactics by George Renko
1600 excercises from CD Killer moves by George Renko
2500 exercises from CD deadly threats by George Renko
1000 x checkmate from CD by Ftacnik
These 16,973 exercises did my rating improve with 170 points. Well because I wasn't improving during the last year I actually have to say that "only" 11,000 problems gave me that improvement of 170 points (65 exercises per point)
When I started with the 3000 problems of the new CD from George Renko "Intensive Course Tactics II" I realized that there was something wrong with my approach.
Investigating this problem I stumbled upon the program of de la Maza.
All of a sudden I realized that I had trusted on the fact that repetition of patterns would come automatically by the sheer amount of exercises.
But that is not true.
My calculation ability has improved much but not my pattern recognition.
So I decided to implement his 7 cycles of repetition.
What I dropped were the micro-drills, the rigid time constraints and the thoughtprocess.
Because I don't think my problem lies there.
Well, I've gathered enough material, so I started over again with the first CD of George Renko, Intensive Course Tactics.
One cycle contains 1355 problems.
I'm now busy with the 5th circle.
I skip the composed endgame problems that are too artificial to appear in a real game.
I intend to do all the CD's of George Renko again 7 times.