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Stata Textbook Examples
Design and Analysis by Geoffrey Keppel
Chapter 4: The Sensitivity of an Experiment: Effect Size and Power

Chapter 4, page 65 shows how to compute omega squared. Stata does not have a built in command for computing Omega Squared, however UCLA Academic Technology Services has created a command omega2 which will compute this for you. If you don't already have the omega2 command, you can download it from within Stata by typing findit omega2 (see How can I use the findit command to search for programs and get additional help? for more information about using findit).
Now that we have the omega2 command, we first run the standard anova as shown below.
use http://www.ats.ucla.edu/stat/stata/examples/da/chap3, clear

anova failures a

                           Number of obs =      16     R-squared     =  0.6473
                           Root MSE      = 12.2661     Adj R-squared =  0.5592

                  Source |  Partial SS    df       MS           F     Prob > F
              -----------+----------------------------------------------------
                   Model |     3314.25     3     1104.75       7.34     0.0047
                         |
                       a |     3314.25     3     1104.75       7.34     0.0047
                         |
                Residual |     1805.50    12  150.458333   
              -----------+----------------------------------------------------
                   Total |     5119.75    15  341.316667 
We can now type omega2 to get the Omega Squared for factor a.
omega2
omega squared = 0.5432

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