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:

- 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.)
- 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.)
- Create a new test by dragging it from the shelf. Change its menu to
**Test Mean**.
- 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.

- 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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June 13th, 2012 → 9:55 pm

[…] Can you do a paired test in Fathom, a normal, orthodox t-test, not this weird bootstrapping stuff? Sure. Check out this post in my other, “Fathom Secrets” blog. […]