The Normal Distribution

Represented by a function/graph. Area under the curve represents the proportions of the observations Total area is exactly 1 (i.e. 100%). How do we locate the median for a normal distribution? The mean? –The median is the value that divides the graph into equal area while the mean is the “balance” point.

The mean is the balance point

The Standard Normal Curve Where is the mean (μ “mu”) and median? What is the standard deviation (  “sigma”  How will the curve change as  changes?

Normal Distributions Symmetric, single-peaked, and mound-shaped distributions are called normal distributions Normal curves: –Mean = median –The mean  and standard deviation  completely determine the shape

The Normal Curve Empirical Rule Discovery

68% of observations fall within 1  of 

95% of observations fall within 2  of 

99.7% of observations fall within 3  of 

Rule Applet

Rule 34% 13.5% 2.35%.15% Applet

Percentiles? 34% 13.5% 2.35% 50 th 84 th 16 th

What’s Normal in Statistics? Normal distributions are good descriptions for real data allowing measures of relative position to be easily calculated (i.e. percentiles) Much of statistical inference procedures area based on normal distributions FYI: NOT EVERYTHING IS NORMAL!

Assessing Normality A distribution is Normal IF: –Histogram creates a symmetrical, bell-shape –Box plot is roughly symmetrical

Distribution of dates is approximately normal with mean 1243 and standard deviation of 36 years

Assume the heights of college women are normally distributed with a mean of 65 inches and standard deviation of 2.5 inches

What percentage of women are taller than 65 in.? %

What percentage of women are shorter than 65 in.? %

What percentage of women are between 62.5 in. and 67.5 in.? %

What percentage of women are between 60 in. and 70 in.? %

What percentage of women are between 60 and 67.5 in? % 13.5% 81.5%

What percentage of women are shorter than 70 in.? % 34% 97.5% 13.5%