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> The only thing you can take away from that is that cracking the joints doesn’t necessarily cause arthritis for him.

It also tells us that cracking the joints doesn't appear to cause arthritis for everybody. If you're a knuckle cracker there's hope.

It tells you that with a sample size of 1. Maybe the person has a genetic mutation that protects against arthritis.

Anecdotes aren't useless, but be very careful of _any_ conclusion you draw from them.