Should I play a risky combination when I don't see the refutation? I've read in a book that one should play a line if (s)he thinks it is decisive and let the opponent proves that (s)he is wrong.

But what about playing a (risky, by risky, I mean that I am not sure) combination when I don't see the refutation?

The trouble I have with sacrificing when I am not sure is that if I am wrong, the opponent will win without effort, just by answering to my moves.

Here is an example of a game that I saw a combination (18. a5), but I didn't play it because I was not sure enough (and it turned out to be good):

[FEN "r4rk1/1b3pb1/pq2pn1p/1pn3p1/PQ1N4/2N1P1BP/1P2BPP1/R2R2K1 w - - 3 18"]

1. a5 Qa7 2. Ndxb5 axb5 3. Nxb5 Na6 ( 3... Qa6 4. Rd6 Nfe4 5. Rxa6 Nxa6 )
4. Qe7 Qc5 5. Qxb7


By the way, my ELO is around 1900.
It depends on how well you like your chances without the combination.

If, taking everything into account (material, position, opponent's rating, etc.), you are likely to win the game in the normal course of events, then play it safe.

On the other hand, if you have lost material, or are in a bad position, or your opponent is rated much higher than you, take a chance.