Psychology 202a
Advanced Psychological
Statistics
September 15, 2015
The plan for today
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Descriptive statistic for kurtosis.
Linear transformations (cont.).
Models.
Conditional distributions.
Numerical assessment of kurtosis
• fourth moment:
x  M   3(n  1)
n(n  1)

4
(n  1)( n  2)( n  3)
s
(n  2)( n  3)
4
• bad idea except in very large data sets
2
Changes in mean and sd under
linear transformation
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Linear transformation: Y = a + bX
New mean = a + b(old mean)
New sd = b(old sd)
Special case: the Z score
Income example
Nonlinear transformations
Models
• What is a model?
• Example: estimating lumber yield
• Why are models useful?
– Models allow us to clarify what interests us.
– Confining our attention to those interests helps
us see structure.
– But danger lurks in those details that we
ignore.