Simply, which of the following two approaches will typically improve tactical ability the fastest:
- Doing puzzles around a known theme or tactical motif - i.e. do 100 puzzles that involve a discovered attack, or a skewer, or mate in 3, mate in 2 etc. etc.
- Doing random puzzles where you don't know the theme or motif behind the puzzle. You may even throw in a number of puzzles without a tactical solution to simply force you think about the next best move.
I always assumed number 2 would be better, but is there any research/authoritative anecdotes/good reasoning that show it is better to do 1?