Module #2, Interactive Exercise #3
If you already have a copy of this page you can go directly to the applet.
Now, move the data points so that you have a correlation of about .00. After getting the correct correlation (or close to the correct value) place a check mark in the box titled Show mean of Y. (Note: Y is the dependent variable).
a. What does the distribution of scores look like? Compare to r = .90 and r = .30.
b. Record the numeric value for SS total here _____________________ (The SS values are shown in the applet.)
c. Now place a check mark in the box titled Show Regression Line. Do your points seem to deviate a lot from the regression line? (Remove the check from the ‘Show SS total’ box and check ‘Show SS error’ to see deviations of observed points from the regression line, shown in red.).
d. Record the SS error here ____________________
e. Compare the error results to the results from the previous problems (r = .30 and r = .90). How do these compare? Compare proportion of error variance for each by taking the SSerror / SStotal.
f. Record SS predicted here ____________________
g. Compare the predicted variance results to the results from the first problem (r = .90). How do these compare? Compare proportion of predicted variance for each by taking the SSpredicted / SStotal.
h. If we don’t know anything about X for a new case, our best prediction of Y is the mean of Y. Does our regression line provide better predictions of Y?
i. Why, when the correlation is .00, does the predicted score match the mean?
j. Does knowing about the relationship between X and Y help us to predict Y in this case?