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 and
high 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.