If this is a live game, I first of all carefully look at the opponent's behavior in order to be sure that he is bluffing. Of course, I take into account the size of my and the opponent’s stacks, and the cards that are on the flop, and how the opponent played throughout the heads-up, how often he bluffed and how often he played in good combinations. All factors must be considered. If this is an online game, then first of all I will look at the statistics of my opponent and then at the size of our stacks.