After you perform an ANOVA, it is useful to be able to report Omega Squared as a measure of the strength of the effect of the independent variable. Stata does not have a program or option that computes this, but ATS has developed a program called omega2 that you can download and use to compute Omega Squared after running an ANOVA. You can download the omega2 command by typing search omega2 (see How can I use the search command to search for programs and get additional help? for more information about using search).
Once installed, you can use omega2 after running an ANOVA. Below, we run an ANOVA where factor b has 4 levels and is highly significant.
anova y b Number of obs = 32 R-squared = 0.8259 Root MSE = 1.21008 Adj R-squared = 0.8072 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 194.50 3 64.8333333 44.28 0.0000 | b | 194.50 3 64.8333333 44.28 0.0000 | Residual | 41.00 28 1.46428571 -----------+---------------------------------------------------- Total | 235.50 31 7.59677419
We can now run omega2 and we get a value of 0.8023.
omega2 omega squared = 0.8023
Below we run a factorial ANOVA, and factor b is again highly significant.
anova y a b a*b Number of obs = 32 R-squared = 0.9214 Root MSE = .877971 Adj R-squared = 0.8985 Source | Partial SS df MS F Prob > F -----------+---------------------------------------------------- Model | 217.00 7 31.00 40.22 0.0000 | a | 3.125 1 3.125 4.05 0.0554 b | 194.50 3 64.8333333 84.11 0.0000 a*b | 19.375 3 6.45833333 8.38 0.0006 | Residual | 18.50 24 .770833333 -----------+---------------------------------------------------- Total | 235.50 31 7.59677419
When there is more than one factor present, you need to supply omega2 with the SS and df for the effect you wish to compute Omega Squared. Below, we compute Omega Squared for factor b by supplying the SS and df for factor b.
omega2 194.50 3 omega squared = 0.8134