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# Aron chpt 3 correlation

## on Jan 27, 2011

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## Aron chpt 3 correlationPresentation Transcript

• Chapter 3
Correlation and Prediction
Aron, Coups, & Aron
• Can be thought of as a descriptive statistic for the relationship between two variables
Describes the relationship between two equal-interval numeric variables
e.g., the correlation between amount of time studying and amount learned
e.g., the correlation between number of years of education and salary
Correlations
• Scatter Diagram
• To make a scatter diagram:
Draw the axes and decide which variable goes on which axis.
The values of one variable go along the horizontal axis and the values of the other variable go along the vertical axis.
Determine the range of values to use for each variable and mark them on the axes.
Numbers should go from low to high on each axis starting from where the axes meet .
Usually your low value on each axis is 0.
Each axis should continue to the highest value your measure can possibly have.
Make a dot for each pair of scores.
Find the place on the horizontal axis for the first pair of scores on the horizontal-axis variable.
Move up to the height for the score for the first pair of scores on the vertical-axis variable and mark a clear dot.
Keep going until you have marked a dot for each person.
Graphing a Scatter Diagram
• A linear correlation
relationship between two variables that shows up on a scatter diagram as dots roughly approximating a straight line
Linear Correlation
• Curvilinear Correlation
Curvilinear correlation
any association between two variables other than a linear correlation
relationship between two variables that shows up on a scatter diagram as dots following a systematic pattern that is not a straight line
• No correlation
no systematic relationship between two variables
No Correlation
• Positive Correlation
High scores go with high scores.
Low scores go with low scores.
Medium scores go with medium scores.
When graphed, the line goes up and to the right.
e.g., level of education achieved and income
Negative Correlation
High scores go with low scores.
e.g., the relationship between fewer hours of
sleep and higher levels of stress
Strength of the Correlation
how close the dots on a scatter diagram fall to a simple straight line
Positive and Negative Linear Correlation
• Use a scatter diagram to examine the pattern, direction, and strength of a correlation.
First, determine whether it is a linear or curvilinear relationship.
If linear, look to see if it is a positive or negative correlation.
Then look to see if the correlation is large, small, or moderate.
Approximating the direction and strength of a correlation allows you to double check your calculations later.
Importance of Identifying the Pattern of Correlation
• A number that gives the exact correlation between two variables
can tell you both direction and strength of relationship between two variables (X and Y)
uses Z scores to compare scores on different variables
The Correlation Coefficient
• The Correlation Coefficient ( r )
The sign of r (Pearson correlation coefficient) tells the general trend of a relationship between two variables.
+ sign means the correlation is positive.
- sign means the correlation is negative.
The value of r ranges from -1 to 1.
A correlation of 1 or -1 means that the variables are perfectly correlated.
0 = no correlation
• Strength of Correlation Coefficients
The value of a correlation defines the strength of the correlation regardless of the sign.
e.g., -.99 is a stronger correlation than .75
• r = ∑ZxZy
N
Zx = Z score for each person on the X variable
Zy = Z score for each person on the Y variable
ZxZy = cross-product of Zx and Zy
∑ZxZy = sum of the cross-products of the Z scores over all participants in the study
Formula for a Correlation Coefficient
• Change all scores to Z scores.
Figure the mean and the standard deviation of each variable.
Change each raw score to a Z score.
Calculate the cross-product of the Z scores for each person.
Multiply each person’s Z score on one variable by his or her Z score on the other variable.
Add up the cross-products of the Z scores.
Divide by the number of people in the study.
Steps for Figuring the Correlation Coefficient
• Calculating a Correlation Coefficient
• Direction of causality
path of causal effect (e.g., X causes Y)
You cannot determine the direction of causality just because two variables are correlated.
Issues in Interpreting the Correlation Coefficient
• Variable X causes variable Y.
e.g., less sleep causes more stress
Variable Y causes variable X.
e.g., more stress causes people to sleep less
There is a third variable that causes both variable X and variable Y.
e.g., working longer hours causes both stress and fewer hours of sleep
Reasons Why We cannot Assume Causality
• Longitudinal Study
a study where people are measured at two or more points in time
e.g., evaluating number of hours of sleep at one time point and then evaluating their levels of stress at a later time point
True Experiment
a study in which participants are randomly assigned to a particular level of a variable and then measured on another variable
e.g., exposing individuals to varying amounts of sleep in a laboratory environment and then evaluating their stress levels
Ruling Out Some Possible Directions of Causality
• The Statistical Significance of a Correlation Coefficient
A correlation is statistically significant if it is unlikely that you could have gotten a correlation as big as you did if in fact there was no relationship between variables.
If the probability (p) is less than some small degree of probability (e.g., 5% or 1%), the correlation is considered statistically significant.
• Predictor Variable (X)
variable being predicted from
e.g., level of education achieved
Criterion Variable (Y)
variable being predicted to
e.g., income
If we expect level of education to predict income, the predictor variable would be level of education and the criterion variable would be income.
Prediction
• Prediction Model
A person’s predicted Z score on the criterion variable is found by multiplying the standardized regression coefficient () by that person’s Z score on the predictor variable.
Formula for the prediction model using Z scores:
Predicted Zy = ()(Zx)
Predicted Zy = predicted value of the particular person’s Z score on the criterion variable Y
Zx = particular person’s Z score in the predictor variable X
Prediction Using Z Scores
• Determine the standardized regression coefficient ().
Multiply the standardized regression coefficient () by the person’s Z score on the predictor variable.
Steps for Prediction Using Z Scores
• So, let’s say that we want to try to predict a person’s oral presentation score based on a known relationship between self-confidence and presentation ability.
Which is the predictor variable (Zx)? The criterion variable (Zy)?
If r = .90 and Zx = 2.25 then Zy = ?
So what? What does this predicted value tell us?
How Are You Doing?
• Change the person’s raw score on the predictor variable to a Z score.
Multiply the standardized regression coefficient () by the person’s Z score on the predictor variable.
Multiply  by Zx.
This gives the predicted Z score on the criterion variable.
Predicted Zy = ()(Zx)
Change the person’s predicted Z score on the criterion variable back to a raw score.
Predicted Y = (SDy)(Predicted Zy) + My
Prediction Using Raw Scores
• Example of Prediction Using Raw Scores: Change Raw Scores to Z Scores
From the sleep and mood study example, we known the mean for sleep is 7 and the standard deviation is 1.63, and that the mean for happy mood is 4 and the standard deviation is 1.92.
The correlation between sleep and mood is .85.
Change the person’s raw score on the predictor variable to a Z score.
Zx = (X - Mx) / SDx
(4-7) / 1.63 = -3 / 1.63 = -1.84
• Example of Prediction Using Raw Scores: Find the Predicted Z Score on the Criterion Variable
Multiply the standardized regression coefficient () by the person’s Z score on the predictor variable.
Multiply  by Zx.
This gives the predicted Z score on the criterion variable.
Predicted Zy = ()(Zx) = (.85)(-1.84) = -1.56