Thursday, March 12, 2015

Remember there is a difference between finding that Z value for a confidence interval and the Z (critical) value for tests and probability.

For example, suppose you want a 95 percent confidence interval for the sample mean.  The formula is mean +/- standard error which is  Z(alpha/2)standard deviation/square root(n).  alpha is 1-.95 = .05. So the Z value is 1.96.

Now if we want an x value that 95% of the data falls below, then we need Z(.05) which is 1.645.

Be careful to understand the difference between the two.

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