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The odds of flopping a set with a pocket pair are about 7.5:1.

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You need better odds for lower pp than higher ones, however, because you are significantly more likely to be facing an overset with 22 than with TT. While your implied odds can make up discrepancies preflop and make it profitable to play 33 with say, 4 other players, hitting your set only to have another player hit another set does tend to reduce this.