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Unless the size of the market is also doubling.

Nope. 50% is 50%. It doesn't matter how big the market is, and that's Norvig's point. Once you penetrate 50% of the market, you're done.

He does admit that you can sell more than one item to the same customer, so you can have > 100% if you compare units sold to potential customers.

Indeed - maybe a corollary should be "in the same colour".

As pointed out by slazaro, this is incorrect. A decrease in usage and/or a change in the market size potentially lowers penetration as a percentage, which could enable it to double again.

As stated, it's either ambiguous or false. If it actually means penetration will never again double from the peak percentage it reached which was >=50%, it's trivial and uninteresting... if it doesn't imply that, then yeah, it's just incorrect.

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