Mary is an old slot-playing friend, one who’s a lot of fun to be around. She never met a good luck charm she didn’t like, and when she heard about my brother Jay’s jackpot dance, she devised one of her own. Hers is better.
“I have a good time,” she said, smiling.
Her cousin takes the slots altogether more seriously.
“She won’t play until she gets a read on the machine,” Mary said. “She asks the attendants which ones have been paying off, then she watches for a while. She wants to see a machine at least giving a little something back to somebody else before she plays. She’s even said, ‘Mary, why don’t you play that one while I watch.’
“Does any of that do any good? Does scouting the slots pay off?” 
No, it doesn’t, I told her. The random number generator turns every play on a slot machine into an independent trial. Results of early spins have no effect on later outcomes. There is no tendency for a hot machine to stay hot, or for a cold machine to stay cold. When a machine has been on a hot streak, all that you can say is that it HAS BEEN hot, not that it’s a hot machine or is likely to continue to be hot.
As for payback percentages you can’t tell just from watching a few spins — or a few thousand — whether a machine is a big payer.
“That’s what I thought,” Mary said. “I’ll go have my fun, and I’ll be her guinea pig for her scouting only if it’s a game I like anyway.”
