Sorry this took so long in queue, got called out for dinner, haven't seen other responses yet...
Of course we can't
know, Laurie

That would take some of the mystery out of it.
Have you ever read the
at casino city times? There are tons of good articles in there. The newsletter is good too (if you don't mind wading through questions you already know the answers to)

Everything onland is pretty well explained there and I've only found one item to dispute (and that was answered, and proven to my satisfaction) here by a member.
There is also onlinecasinocitytimes, but I find more valuable information right here.
"Can online casinos tighten or loosen the slots?"
Depends on the situation:
Yes. Do they? Mostly no.
From what I've gathered here:
Microgaming - No. No individual operator can adjust them. They don't speak openly anywhere, but we have been assured by Ed Ware, webcaz, and possibly others that the payout percentages are simply set, and the return to player (rtp)% is the same across the board.
Granted, they don't inform us, or anyone else when they change it, but there is no reason to believe they would.
Debatable... if MG gets a 'cut' of all loss action, or if whitelabel or other affiliates now own MG - alll bets are off on their integrity.
RTG - Yes. As far as we know, the software can be tweaked by the operator. (We don't know if the rtp can be reset across all operators by RTG themselves) We've been told by Z, a former software person for them, that operators can't adjust it to 'anything crazy like 80%' (no cite, just memory). And we know from former RTG managers here that it is 'casino-wide' when they adjust it, not account by account.
Rival - Yes. They change their paytables occasionally. Thye don't 'alert' everyone who plays there, but their paytables are available from within the 'help' tab when the game is logged in.
Other 'major players'? I dunno. Software is software... if they seem iffy in any other respect, might as well assume the worst until they assure us (and can back it up with independent audits)
3Dice assures us that their game is fair. There is no reason in my mind to doubt it. We were assured that external auditing is on their agenda. I would be more leary of the ability of a semi-independant casino to skew the odds than a casino breaking into online with all of their online eggs in one basket.
It all boils down to trust.
Half-ass 'regulated' casinos outside of nevada (possibly AC and tunica?) can and will do what ever they please. But John G. at casinocity times has explained the shear improbability of nevada gaming commision casinos 'flipping the switch'. The throughput money lost in the downtime (there's a waiting period after a machine is changed, whether the face is opened or it is done from kiosk) just doesn't add up to a minor adjustment.
But keep in mind the bottom is 75% and the top is wherever they want to set it. so, yeah, I might argue with John.
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Did I answer your question, Laurie? At least I gave you some resources, and if no one else has replied yet (been typing and eating and drinking and watching teevee for a while now

) they should. It isn't a tired question, it's a damn good question.