Bell-shaped symmetrical frequency distribution
curve. It is characteristic
of many economic, natural, social, and other real world phenomenon (such as IQ scores, height variation
within a population
, weights of crop yields, variation in quality
of manufactured goods) where two or more variables have direct relationship
predictability (low variation). In normal distribution
, extremely-large values
and extremely-small values are rare and occur near the tail
ends. Most-frequent values are clustered around the mean (which here is same as the median
and mode) and fall off smoothly in either side of it.
In normal distribution, 68 percent
of all values lie within one standard deviation
, 95.45 percent within two standard
deviations, and 99.8 within three standard deviations (called six sigma
in quality control). In other words, only one out of a thousand values will fall outside of six sigma. This distribution is called 'normal' in the sense of 'ideal' or 'standard' against which other distributions may be compared. Also called Gaussian distribution.