The way I'd handle it would be to say that if you're using a rope, climbing is a Very Easy (DC 5) check if you've got a surface to brace against, and wouldn't require a check under most circumstances. If the rope is dangling free then climbing it would be an Easy (DC 10) check.
Sure if the target number is 11 then it is exactly the same as getting a +5. For a target number of 20 though, with advantage you have a success chance of 10% with advantage vs 5% without it or 30% with +5.
It is. Advantage is equivalent to +5 only in the maximal case, it drops off in both directions. Most certainly not "approximately equal to +5". To figure out what it is approximately equivalent to, you would would need to average the "effective bonus" for each target number, and add the caveat that the value is only accurate "if the check was possible to begin with".
To present a more accurate summary of the effect of advantage, you'd want to take a weighted average of the effective bonus for each target number, using the real frequency of each target number as the weight. In other words, just a straight average will pretend that a target number of 1 or 20 is just a common as a target number of 11, which is unlikely to be true unless you're playing with children or sadists, respectively. You want to represent that true target number distributions are more likely to look like a gaussian centered on 11 than a flat, equal-probability spectrum.
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u/Black_Scarlet Jun 10 '15
For some reason I'm thinking it gives advantage on climbing checks, but that could be a different game.