Normative Rules of Probability Calculus
- Probabilities are always positive, or equal to 0.
- The probability of an event plus the probability of its opposite must
always equal 1 (because the event either happens or it doesn't.)
- The probability of an event A is always greater than or equal to the
probability of an event A and an event B.
For example, if you choose a person at random, you have a better chance of
choosing a woman than you have of choosing a woman with dark hair.
Normative Rules for Statistics (Sampling)
- The more people we survey, the more accurate the result. Larger samples
are better. (This is known as the Law of Large Numbers.)
- Random processes are not self-correcting.
For example, if you do a survey of children's intelligence and the first
child you survey happens to have an IQ of 150, the result will be higher
than average. Many people think that, if you survey 9 more children, the
sample will still give average intelligence -- but this is simply not true.
Why should the next 9 children surveyed all have lower than average IQs?
Numbers for the real world: The numeracy page.
Some guidelines for 'quick check' calculations.
Some useful number facts for quick checks.
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