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# Crosstabs - PowerPoint PPT Presentation

Crosstabs. How do we assess the relationship between two variables? (We’ll bring in more variables later.) Various ways, especially with interval-level data; one of the most common ways is with crosstabs. “Crosstab” is a contraction of “Cross Tabulation”

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Presentation Transcript

• How do we assess the relationship between two variables? (We’ll bring in more variables later.)

Various ways, especially with interval-level data;

one of the most common ways is with crosstabs.

• “Crosstab” is a contraction of “Cross Tabulation”

Also called a contingency table

• What is a (simple) crosstab?

A table based on two variables, where the cell

entries are the counts or percentages of cases

that fall in that row or column category.

So, also called a bivariate frequency table.

• WARNING:

• There’s going to be a lot coming at you (in class).

• It requires paying attention, thinking (wow!).

• But—it only involves percentages, so no complicated “statistics” (yet).

A file of about a thousand people would have data like this (except that Gender and Watching TV would be coded using numbers).

You would create the cross-tab (presumably using SPSS—these are real bears to do by hand).

An example: gender & tv watching

• You might want to ask the question: are women more likely to watch this particular tv program than men are?

• So, you display the data in a crosstab (in this case a 2x2 table).

• But how do you read it?

We can % either way rather than numbers of cases. (by rows) (by columns)

• Note rather than numbers of cases.

I’ve percentaged the total row or col.

Not necessary, but often useful, and

it’s done automatically by SPSS.

I’ve used whole numbers. Nothing

accuracy level. Don’t overdo

accuracy.

• Which way is correct? rather than numbers of cases.

• Recall:

Dv is the one we’re trying to explain.

Iv is the one used to explain the dv.

The question we are asking is: are

women more likely to watch this

particular tv program than men are?

Burning question rather than numbers of cases.

• Does it make any difference which variable makes up the rows, and which the columns?

• No. There’s no agreed-upon convention for whether the iv goes in the rows or columns (text notwithstanding). BUT

• If you switch row and column variables, then which percentages are right (for your question) will also change.

More analytical matters. rather than numbers of cases. Illustration: Do different careers attract different partisans? (class survey)

• Are Democrats or Republicans more likely to go into:

• Law?

• Politics?

Party ID * Career rather than numbers of cases.

Recode? Note small # of cases in some rows. Also, are Ind and Other different?

DKs? Delete or combine with other rows?

More analytical matters (cont.) rather than numbers of cases.

• These are analytical matters.

Don’t make meaningless combinations

just because of small N’s.

Keep in/delete “don’t knows” depending

• Suppose we decide to keep DKs and combine the three smallest categories.

Recoded rather than numbers of cases.

• Table is simpler, easier to read.

• More meaningful because it doesn’t make distinctions we aren’t really interested in.

Final analytical matter rather than numbers of cases.

• How much of a difference is enough to be meaningful?

Important question

For now, see Weisberg et al. reading,

pp. 211-12.

You might want to look at this when

Presentation matters rather than numbers of cases.

• DO NOT USE SPSS OUTPUT DIRECTLY

Reformat as necessary

Provide meaningful labels

Give it a title

Show n’s, %s, not cell counts

• Can be easily done in MSWord (maybe other ways as well)

Data Analysis #1 rather than numbers of cases. Due one week from today (by ind’s, not pairs)

• Directions are on the syllabus

• Describe a hypothesis (NES or GSS).

Tell us why this hyp makes sense.

Thoughtfulness is rewarded. (Dem’s

more often voted for Gore is not that

thoughtful.)

• Discuss the opera’zation of your concepts.

Tell us how you operationalized your

variables, but also why.

• Generate an SPSS cross-tab to test your rather than numbers of cases.

hypothesis.

Percentage the table properly.

Presentation as noted earlier.

• Explain the table: is your hypothesis

supported?

(More than “yes” or “no” is required.)

Note possible alternative explanations.

• Usually ≤ 3 pp. (double-spaced) + table rather than numbers of cases.

Table should go on a separate page.

• Writing is important.

Use clear, straightforward prose (you

are not writing a novel).

Proper grammar; correct spelling,

punctuation, and capitalization; typo-

free