That does not make sense. Then a keeper who faces the second penalty in his career has a success rate of either 100% or 0%. But nobody would use it as an argument because the number is too small. The number is still too small at the end of a keeper's career to truly statistically prove anything regarding a penalty saving ability. It's very simple.
I didn't say it depends only
on the success rate. But come on, you talked about statistics. If you have studied the subject even a little bit, you know it makes no sense to say that the kind of sample size goalkeepers face over their career (for a long career probably somewhere between 50-100) cannot possibly be statistically significant. It can, if there is enough difference in the success rate. That's how statistics work, and indeed it's quite simple. Enough difference in the success rate in a big enough sample, and we can say there's a statistically significant difference.
Because there is no one penalty saving ability per se. It is a combination of anticipation and reflexes on the line. Both are already judged by whether or not a keeper is better than another keeper. If it was practicable to the point of making a difference you could have keepers who save it most of the time. But that is not possible because it is not practicable. And that is because it simply depends on the shooter shooting it right or not.
I don't get this logic at all. Why would keepers have to save penalties most of the time if it's practicable? If they get their save rate up from 10 to 20 percent by practicing, why wouldn't this show it's practicable? And it doesn't "simply depend" on the shot. You're thinking about this in yes/no manner when it's clearly more complex. Yes there are penalties that are unsaveable. And there are penalties whose savability is only down to luck. But there's also the third category where a good penalty saver can make a difference. To be sure, the difference probably isn't much. But I see no reason to believe it doesn't exist.
Finally, I don't agree that penalty saving only demands the same qualities as goalkeeping in general, either. I've heard many goalkeepers talk about the way they observe the shooter's body position, eyes, etc. to predict where they're going to shoot. This kind of a skill sounds to me penalty-specific. Of course there's limits to how much that can be done, but again it would be weird to think that no difference at all can be made.