How many parameters are needed?

With four parameters I can fit an elephant, and with five I can make him wiggle his trunk

John von Neumann

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Where did the famaous 5% come from?

If one in twenty does not seem high enough odds, we may, if we prefer it, draw the line at one in fifty (the 2 per cent. point), or one in a hundred (the 1 per cent. point). Personally, the writer prefers to set a low standard of significance at the 5 per cent. point, and ignore entirely all results which fail to reach this level. A scientific fact should be regarded as experimentally established only if a properly designed experiment rarely fails to give this level of significance.

Sir Ronald A. Fisher