A.P. Psychology
Statistics Notes
Correlation
 The way 2 factors vary together and how
well one predicts the other
 Positive Correlation- direct relationship
where factors increase or decrease together
 Negative Correlation – Inverse relationship
where one factor goes up while one goes
down
Continued…
 Illusory Correlation
-perceiving correlation when none exists
-we notice random coincidences as not
random, rather as correlated
Statistical Reasoning
 Mode – the most frequently occurring score
in a distribution
 Mean – the arithmetic average of a
distribution
 Median – the middle score in a distribution
Measures of Variation
 Range – the difference between the highest
and lowest scores in a distribution
 Standard Deviation – A computed measure
of how much scores vary around the mean
score
 Statistical Significance – A statistical
statement of how likely it is that an obtained
result occurred by chance
68-95-99.7 Rule
 In practice, one often assumes that the data are
from an approximately normally distributed
population.
 If that assumption is justified, then about:
– 68% of the values are within 1 standard deviation of
the mean
– about 95% of the values are within two standard
deviations
– and about 99.7% lie within 3 standard deviations.
68-95-99.7 Rule
Skew
 A distribution is skewed if one of its tails is longer
than the other.
– Positive Skew - long tail in the positive direction.
– Negative Skew - long tail in the negative direction.
– No Skew – symmetric, normal distribution.
 Distributions with positive skew are sometimes
called "skewed to the right" whereas distributions
with negative skew are called "skewed to the left."
Positive Skew
Negative Skew
No Skew (Normal Distribution)