Productive tactical puzzles: random or themed? 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?
In my experience if you are starting out it is better to solve same thematic tactical problems in a series and slowly moving onto other themes. After you get accustomed with the themes random problems or problems which doesn't tell you what to do helps.