Odds of a major downswing?

[Pokerscott]

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That would tell me my % chance of having a 10 buyin drop,

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You didn't understand me. I was suggesting you calculate the odds that a 15% player will have a -10 buyin drop over 300 sngs, within his 1800-1900 sng sample. You might be 1% or 5% to experience -10 buyins over 300 sngs, but you have played 1800-1900 and not 300, which makes this much more likely.

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Exactly.

Flip a coin 6 times. Chance you get six straight tails is 1/64.

Flip a coin 100 times and the chance you stop somewhere along the line and post "Hey! I just got 6 tails in a row!" much much higher.

Here is a link to a post where I put some tables that answer your question if you are a 10% ROI or 20% ROI. I didn't do the 15% case though.

ROI variance tables

Pokerscott