4.5 Normal distribution of measurements

Many traits of animals show a normal distribution. This means that the distribution is symmetric and can be characterized by a mean and variance. Below and above this mean you see the same number of animals. And by the fact that further away from the mean you see a decreasing number of animals with this measured unit. Graphically it can be expressed as:                         

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In such a normal distribution you see that 68 per cent of the animals has a value for this trait within one standard deviation from the mean; 95 per cent within two standard deviations from the mean and 99.7 per cent within three standard deviations from the mean.