Oh i agree that things like curve and itemlevel (and raider io and wowprogress and such) are very poor predictors of performance (whether performance just means dps/hps, or some more refined definition that includes use of interupts/damagereductions and other utility when appropriate, or the ability to adjust to a sudden death/gamechanger, whatever).
But the kind of alternative manual vetting you’re describing hardly happens anymore. That’s what people did when they had to spend a good hour or more putting their group together using trade chat, going by reputation, vouching or by a little interview.
Be it sad or not, that doesn’t really fit the way most people raid now, certainly not spontaneous pug groups anyway.
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When you (as a group leader) are just going to take a gamble on people to fill your group quickly, then it makes sense to filter by item level and curve (and lean on flavor of the month classes) just to at least have a healthier pool to fish in.
Not because they’re good indicators of skill or outcome, but because the percentage of players who know what to do is higher among the have-curve group than among the dont-have-curve group.
That doesn’t say anything about any individual player but it is statistically a no brainer. Akin to betting on (random color, black or red) instead of on (random number) in roulette (ok fair enough, that is a much more extreme 49% vs 3% example, but you get the idea).
Put another way, there are plenty of bad players with high itemlevels, and plenty of great players with crap itemlevels, but if you ambush 100 randomly selected ilvl 360 players with a popquiz on UU’nat-mechanics , and do the same to a 100 randomly selected ilvl 415 players then the average score will no doubt be better among the 415 group.
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Anyway, as for your situation, i get that having a volatile workschedule takes the typical planned guild raids off the table. But you can probably find some big guild or big community that just has a whole bunch of people that form raids every evening at X o’clock among themselves (no pre-signups, no attendance requirements, etc).
I don’t raid at all myself at the moment so i’m afraid i can’t really throw out a link to such a community, but i see such groups scroll by quite often here and there on the forum or on wowhead or on discord.