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Density Curve
 Mathematical model of a distribution
 Total area under the curve is equal to 1, or 100%
 Normal distributions: symmetrical, bell shaped density curves defined by a mean and a
 standard deviation.

Emprirical Rule
 About 68% of all observations are within 1 standard deviation
 of the mean.
 About 95% of all observations are within 2 standard deviations of the mean.
 Almost all (99.7%) observations are within 3 standard deviations of the mean.

zscore
 Standardize our data to transform any Normal curve N (μ, σ) into the standard Normal
 curve N (0,1).
 A zscore measures the number of standard deviations that a data value x is from the
 mean μ
z = (x µ) / σ
 When x is larger than the mean, z is positive.
 When x is smaller than the mean, z is negative.

Standard Normal Probabilities
 Table A gives the area under the standard Normal curve to the left of any zvalue.
 When you know the proportion, but you don’t know the xvalue that represents the cutoff,
 you need to use Table A backward.

