# Download Lecture 3 -- Central Tendency and Variability

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```Stats 95
•
Mean
• μ = population mean (“MU”)
•
•
•
•
M or
= sample mean
∑ = Sigma, summation (“add all of these”)
N = population size
Later… n = sample size
X

N
Variance & Standard Deviation
SD2 or σ2 = Variance
 X   
 
N
SD or σ = Standard
 X   
 
N
Deviation
2
2
2
Z-Scores: The Standard Deviation
“Meter”
 X   
 
• Use Z-scores to
express values
regardless of the
original unit of
measure
• E.g., feet or meters
• Once you have the
standard deviation, you
can go from raw scores
to z-scores, and from
z-scores to raw scores.
N
z
( X  )

X  z   
2
Normal Distributions
```
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