Paired Tests in Fathom

Posted on June 8, 2012

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This is one of those, “oh, yeah” and slap your forehead things.

Suppose you have data on which you want to do a paired test. The prototypical paired test is a pre-post test of some kind. Let’s call these scores pre and post.

The basic idea is this: compute the pre-post change and test whether the mean of this change is different from zero.

Assuming you’re testing whether there’s a change, here’s what you do step by step:

  1. Make a new attribute, perhaps called change. (An attribute in Fathom is probably known as a variable most places. It’s a column in the table.)
  2. Give it a formula: post – pre. (Right-click on the column heading change and choose Edit Formula. Enter the formula in the formula editor that appears.)
  3. Create a new test by dragging it from the shelf. Change its menu to Test Mean.
  4. Drag change to the space at the top. Done!

Of course, if you presumed ahead of time that the scores should go up, you do a one-tailed test.

  1. Click the phrase “the mean of change is not equal to 0″; from the menu, change it to “the mean of change is greater than 0.”

Sample table showing fake data. Show Formulas is selected.

The test (this one is one-tailed; see the blue “greater than” text?). Also, see “change” at the top after Attribute:Numeric. That’s where we dragged the attribute.

If for some reason you want to test that the mean of change is some number other than zero, just edit the number “0” in the blue text.

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Posted in: How-to