The Normal Distribution, also known as the "Gaussian Distribution" or "bell-curve", is the most widely
employed function relating observations X with probabilty P(X) in statistics. Many natural populations
are approximately normally distributed, as are several important derived quantitities even when the
original population is not normally distributed.
P roperly speaking, the Normal Distribution is a continuous "probability density function" meaning that
values of a random variable X may take on any numerical value, not just discrete values. In addition,
because the values of X are infinite the "exact" probabiliy P(X) for any X is zero. Thus, in order to
determine probabilities one typically looks at invervals of X such as X >2.3 or 1< X < 2 and so forth. It is
interesting to note that because the probability P(X) = 0, we don't have to worry about correctly
interpreting pesky boundaries, as seen in discrete distributions, since X > 2 means the same thing as X 2
and X < 2 is the same as X 2.
As described previously, the Normal distribution N(, 2) consists of a family of curves that are specified by
supplying values for two parameters: = the mean of the Normal population, and 2 = the variance of
the same population.
Prototyping the Normal Function using the Gaussian formula:
Making the plot of N(50,100):
< specifying mean (
50
2
i 0 100
< Defining a bunch of X's ranging in value from 0 to 100. Remember that the
range of X is infinite, but we'll plot 101 point here. That should give us enough
points to give us an idea of the Gaussian function shape!
X i
i
Y1
i
< specifying variance ( 2)
100
100
1
2
1 X 2
2 i
2
e
< Formula for Normal distribution. Here we
have computed P(X) for each of our X's.
Zar 2010 Eq. 6.1, p. 66.