Math 104

April 8, 2005

SPSS Assignment 7  (due April 15)

 

Read most of Chapter 20 in the SPSS text:  pp. 435-454.

 

Turn in these problems:

            Chapter 20 Concepts (pages 462-465): Problem 1acd, 6.

            Chapter 20 Data Analysis (pages 465-469): Problems 1abcd, 2abcde, 3ab, 5abc, 9.

 

 

 

HINTS AND SUGGESTIONS

 

For the reading: 

 

            This is the SPSS text’s section on regression lines.  The SPSS text usually calls them “least-squares lines,” but that’s a synonym for regression lines.

 

            There are some other differences in style.  For the example, the SPSS text uses “y = mx + b” for the equation of a line, while we usually used “y = bx + a.”  That would be less confusing if the same symbol “b” weren’t being used in two different ways.  Unfortunately, those are the two forms most commonly used.

 

            The SPSS text talks a lot about “R square” or R2.  The “R” is the same thing as our correlation coefficient, r.  They’re squaring it because for some purposes, they want it always to be positive.  Both forms are widely used.  SPSS will calculate R2 for you, but if you want R (or r) you have to either

            (a) Take the square root of R2 by hand, and guess the sign, or

            (b) Get SPSS to print out a “Model Summary” table and look for r in it.

 

            On page 443, there’s a formula for the slope that looks harder than the one you’re used to.  That’s because the SPSS text assumes that you don’t know how to compute r.  Essentially, they’re doing it inside the formula.  Later, on page 446, they get  r  from the slope by using your formula backwards.  (Fortunately, you may never have to any either of these formulas again.)

 

            The reading gets to be hard going, especially around page 450.  If you must skip stuff, jump to “Some Warnings” on page 451.

 

For the “Concepts” questions:

 

Problem 1:  Part (b) (the “intercept” or constant term) is optional.  Do it if you think it helps you.  You might be able to answer the other parts without using the intercept.

 

If you’re doing this in a Word document, feel free to draw the figure by hand.

 

Problem 6.  The point is that regression only works for numerical variables.  (And only real numerical variables—if numbers are used as meaningless labels, that isn’t good enough.)

 

For the “Data Analysis” questions:

 

Problem 1:  Make sure that “husbeduc” is the dependent variable, on the vertical or Y axis. 

“wifeduc” belongs on the X axis.

 

To draw the line, you’ll need the Chart Editor.  Look for the “Fit Line” box, and select “linear regression.”  Check both the check boxes.

 

For problem 1c, you can find exact values in the “Coefficients” table (see next problem).  The value of R (for 1d) is in the “Model Summary” table (also see next problem).  If you can find these tables, copy them into your word-processor document.

 

Problems 2d and 2e:  These probably need to be done by hand, but you may be able to get SPSS to give you some of the needed information.  Hint on d:  If the husband’s years of education were always exactly the same as the wife’s years of education, what would be the equation describing their relationship?  Hint on e:  If wife’s education were really useless for predicting husband’s education, then what value would you always guess for husband’s education?

 

Problem 3 is like problems 1 and 2, but you need to switch the X and Y variables.  That means starting over with the regression — almost nothing is salvageable when you make the switch.

 

Problem 5a.  “wifeft” becomes the “Set Markers by” variable.

 

Problem 5b.  In  order to draw separate regression lines and get separate R2 values, do the following:

            Open the Chart Editor

            Click on “Options” under the “Chart” menu

            Find “Fit Line” in the Options Box, and select “Subgroups”

            Click on “Fit Options..”.

            Click on “Display R-squared in the legend” and “Linear Regression”.

            Click Continue and OK (but keep the Chart Editor open for now).

 

Unless you’re printing in color, you’ll have to do something to make the lines look different from each other.  In the Chart Editor, click on one of the lines and look for the “styles” options.  There’s lots of room to be artistic here.  You can probably change the style of dots in your scatterplots, too, if color isn’t enough.

 

Problem 9.  “salnow” is the Y variable, “salbeg” is the X variable.  (That’s the same way as in SPSS-6, but it’s more important now.)

 

 

(end)