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In a binomial experiment, if we observe $x=0$ positive individual among $n$ individuals, then the proportion of positive individuals is significantly lower than $3/n$ with a type 1 error less than and very close to $5\%$. This fact, sometimes called the "rule of three", is a consequence of the inequalities $$\exp\left(-\frac{np}{1-p}\right) \leq \Pr(X=0) \leq \exp(-np).$$

Do you know other such basic easy rules for statistics? I find them very interesting and useful. This principle is not really a "rule of thumb" because it has a reliable theoretical foundation, but I don't see another tag for this question (I hope it is not off-topic)

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  • $\begingroup$ "Normally, more than two-thirds of the people are average" (meaning within one standard deviation of the mean)? $\endgroup$ – Dilip Sarwate Jun 8 '12 at 11:42
  • $\begingroup$ One very simple one that comes to mind is how the variance of a sample proportion of successes out of $n$ Bernoulli trials is no more than $1/4n$ (which is achieved when the success probability is $1/2$) $\endgroup$ – Macro Jun 8 '12 at 12:11
  • $\begingroup$ Many "rules of thumb" are based on theoretically rigorous analyses or approximations. $\endgroup$ – whuber Jun 8 '12 at 13:02
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Check out Gerald van Belle's book "Statistical Rules of Thumb" a very nice little paperback text loaded with examples of rules of thumb and explanations including the "Rule of three" that you mention above.

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  • $\begingroup$ (+1) That seems like a nice book. Not only does he give rule-of-thumbs, but he also motivates them and discuss their validity. $\endgroup$ – MånsT Jun 8 '12 at 12:13
  • $\begingroup$ Nice! I will ask to my boss to buy it :) $\endgroup$ – Stéphane Laurent Jun 8 '12 at 12:13
  • $\begingroup$ Ahah ! I've just send an email to my boss and actually we already have this book :) $\endgroup$ – Stéphane Laurent Jun 8 '12 at 12:20
  • $\begingroup$ Yes, it's a great book. It's fun just to read it. $\endgroup$ – jbowman Jun 8 '12 at 13:28
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    $\begingroup$ stats.stackexchange.com/questions/2715/… $\endgroup$ – Stéphane Laurent Jun 8 '12 at 14:35

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