Page 179 - Statistics II for Dummies

Chapter 9: Testing Lots of Means? Come On Over to ANOVA!
You have to carry out three major steps in order to complete the F-test. Note: 163
Don’t get these steps confused with the main ANOVA steps; consider the
F-test a few steps within a step:

1. Break down the variance of y into sums of squares.
2. Find the mean sums of squares.
3. Put the mean sums of squares together to form the F-statistic.

I describe each step of the F-test in detail and apply it to the example of com-
paring watermelon seed-spitting distances (see Table 9-1) in the following
sections.

Data analysts rely heavily on computer software to conduct each step of the
F-test, and you can do the same. All computer software packages organize
and summarize the important information from the F-test into a table format
for you.

This table of results for ANOVA is called (what else?) the ANOVA table.
Because the ANOVA table is a critical part of the entire ANOVA process, I
start the following sections out by describing how to run ANOVA in Minitab
to get the ANOVA table, and I continue to reference this section as I describe
each step of the ANOVA process.

Running ANOVA in Minitab

In using Minitab to run ANOVA, you first have to enter the data from the k
samples. You can enter the data in one of two ways:

✓ Stacked data: You enter all the data into two columns. Column one
includes the number indicating what sample the data value is from (1 to
k), and the responses (y) are in column two. To analyze this data, go to
Stat>ANOVA>One-Way Stacked. Highlight the response (y) variable, and
click Select. Highlight the factor (population) variable, and click Select.
Click OK.

✓ Unstacked data: You enter data from each sample into a separate
column. To analyze the data entered this way, go to Stat>ANOVA>One-
Way Unstacked. Highlight the names of the columns where your data are
located, and click OK.

I typically use the unstacked version of data entry just because I think it
helps visualize the data. However, the choice is up to you, and the results
come out the same no matter which method you choose, as long as you’re
consistent.

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