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Final

Department

PsychologyCourse Code

PSYB07H3Professor

Douglas BorsStudy Guide

Final AFTER MIDTERM EXAM NOTES

PEARSON-PRODUCT MOMENT CORRELATION COEFFICIENT (r)

-tells us the strength of the relationship between x and y

-the average product of z-scores same thing as z scores

-correlation coefficients are the covariance when there are standard

variables standardize x but leave y and its use get a measure in y

units

COVxy = a measure of the relation between x and y the covariance

standardized by the standard deviation of x and y

-to standardize , we divide the covariance by the size of the standard

deviations.

-g

iven that the maximum value of the covariance is plus or minus the

product of the variance of x and the variance of y, it follows that the

limits on the correlation coefficient are +1.0 or – 1.0

-the correlation coefficient is not an unbiased esimator

Example:

rC O V

s s

x y

x y

=

41347

01116

11205

11014

4943

x

s

y

y

=

=

1 1

1 5 8.

x- y-

If the regression coefficient is computed

the slopes can be he same but the

correlation is different i.e. the second

scatterplot has more noise.

1.

2.

3.

-expected value of r is not (rho --> Greek letter), then we correct for itρ

-correlation cannot = 1 because there will be many variables that affect

(influence) relationship of behaviour you’re trying to predict

r2 = that proportion of the variance is y that is shared (accounted for) by

x. Sometimes called “coefficient of determination”

-therefore, r = 0.9 and r2 = 0.81 or x account for 81% of the variance in

y (doesn’t mean x CAUSES y to chance…it covaries

-i.e. r =0.2 thus r2 = 0.04 or 4%

-i.e. r = 0.4 thus r2 =0.16 or 16%

-if our r is g times as large as a second r, then the proportion of the

variance associated with the first r will be g(squared) times as great as

that associated with the second.

-the chance of a zero slope is slcose to zero

-you must ask how reliable is the relationship between x and y

Factors Affecting r

-correlation tells us about the relationship between the variance of x and

variance of y and what other factors affect y

1. Range Restrictions

x

s

x

x

=

=

5

1 5 8.

C O V

x y

=

2 2 5.

rC O V

s s

x y

x y

=

r

r

=

=

2 2 5

1 5 8 1 5 8

0 9

.

( . ) ( . )

.

rr N

N

adj

= − − −

−

11 1

2

2

( ) ( )

( )

r

a d j

= − − −

−

11 8 1 1

2

( . ) ( 5 )

( 5 )

= .75

-you get a circle which can show that there is no relationship between x

and y (when there might be)

2. Outliers

-leads to a big z-score can create illusion of a strong/weak correlation

when there isn’t

3. Heterogeneous Subsamples

-blue: strong moderate correlation

-green strong correlation

-put blue and green together get a weak relationship weak

correlation

-indicates that you shouldn’t put two sub groups together, can be

misleading

Whole Part Correlation produces a bias

-this is where the score for variable x contributes to the score of variable

y produces a positive bias in r

-again, correlation does not imply causality variables may be

accidentally related or both may be related to a third variable or thy

may influence each other

i.e. the price of petroleum is correlated with Bors’ age but it doesn’t

mean the price is going up as Bors ages or because Bors ages it is

accidentally correlated

-what is more informative: the slope of the regression line or the

correlation coefficient? they both answer different things

Additional Notes

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###### Document Summary

Tells us the strength of the relationship between x and y. The average product of z-scores same thing as z scores. Correlation coefficients are the covariance when there are standard variables standardize x but leave y and its use get a measure in y units. Covxy = a measure of the relation between x and y the covariance standardized by the standard deviation of x and y. To standardize , we divide the covariance by the size of the standard deviations. G iven that the maximum value of the covariance is plus or minus the product of the variance of x and the variance of y, it follows that the limits on the correlation coefficient are +1. 0 or 1. 0. The correlation coefficient is not an unbiased esimator. If the regression coefficient is computed the slopes can be he same but the correlation is different i. e. the second scatterplot has more noise. 8 r a r a j j d d.

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