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Hypothesis Testing II
The t - Distribution

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*Sign up* Assignment
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Introduction
In behavioral research, we often have to estimate the standard deviation
of the population from sample data.
We assume that the sample data provides a reasonable estimate.
However, we are not likely to get extreme scores in our sample.
To compensate, we calculate the SD differently when we want to get
an estimate of the population standard deviation.

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*Sign up* Comparing Calculations
The Standard Deviation for a population:
σ =
Square root of
SS/N
The estimated Standard Deviation for a population:
s = Square root of
SS/n-1

A New Probability Distribution
The Unit Normal Distribution is a population distribution.
Requires that the population standard deviation is known OR
Requires a very accurate estimate of the population standard
deviation (n > 45).
When we estimate the population standard deviation from small samples
we must use the t – distribution rather than the Unit Normal
Distribution to find probability.

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