Computer evaluations: How trustworthy are they? Fritz 12 with Rybka gave a friend of mine an evaluation of +3 for white in this endgame position, 

[Title "White to move"]
[FEN "8/8/8/3k3p/3P3K/2P3R1/8/7r w - - 0 1"]


which turned out to be a draw. But I've heard that +3 from a computer means a guaranteed win with perfect play. Did I just hear wrong? How should computer evaluations be interpreted in general? What do opening advantages of &lt;.5 even mean?
Different engines have different "scales" for their numerical evaluations. For instance, in a typical middlegame position with plenty of play left, when Houdini says +2.00 or better, it is highly probable that White has a winning advantage (though even here I've included qualifications for a reason). But consider: one could modify the source code of Houdini and double the absolute values of all numbers involved in evaluations; one gets an engine of identical strength that produces identical play, but now +4.00 means what +2.00 used to mean. This illustrates that one shouldn't expect a uniform numerical threshold across engines that typically indicates a winning advantage.

More than this, though, it's important to understand that a numerical engine evaluation of a position (as opposed to an outright declaration of inevitable mate) never strictly translates to "a won game," even for a single, fixed engine. A key point is that numerical evaluations have no clear-cut "meaning" in broad chess terms, and are rather just a substitute for sentient thought that is used to mechanically guide an engine toward generally desirable outcomes by influencing which move it selects at each point in the game; in this light, what is ultimately most important to an engine's play is just the difference in evaluation assigned to potential moves, rather than anything about the absolute values involved. The numbers are useful to the engine itself, which needs something that concrete in order to make a decision for one move over another, but we humans shouldn't be too quick to read more meaning into the magnitudes involved with thoughts like "+X means a win."

In particular, the further and further we get toward an endgame as opposed to a middlegame, the less we can use a rule of thumb (like my +2.00 for Houdini in middlegames above) about a certain threshold being enough for a win. One key reason for this is the difficulty that engines have recognizing fortresses, where an abundance of extra material still isn't enough to win. For instance, when I feed Stockfish this position,

[FEN "5k2/6p1/5r2/3Q3P/4K3/8/8/8 w - - 0 1"]


after a couple minutes thought it is giving an evaluation of about +7.00, and in a typical position, when Stockfish says that, you almost certainly do have a win on your hands. Nevertheless, this is a dead draw, and a human can see this easily once the fact is realized that Black can just shuffle the rook between f6 and h6, and so (1) the h-pawn is useless, and (2) the white king will never be able to help the white queen attack. Eventually, Stockfish will recognize a draw here too, once it butts up against 50 moves, say, or finally runs out of different moves to try and finally can't avoid a repetition, but those events are way down the search depth line.

The endgame position from your earlier question that you linked to is akin to this sort of fortress, in that the extra connected passed pawns White has there are nice and all, but ultimately not quite enough to win in that position. If an engine were to calculate for enough time to see as much information as is contained in tablebases, then its evaluation would come down to 0, but in the meantime, its evaluation algorithm has nothing better to go on than to give a + for that extra material (that it doesn't yet know is meaningless).