Eta-Square ( 2)
A calculation in the t-test family that is analogous to r2 in correlation and regression is eta-square ( 2). This number can be interpreted in exactly the same way as r2. Therefore an 2 = .367 means that 36.7% of the variability in the dependent variable can be explained or accounted for by, the independent variable. Eta-square should be routinely calculated for t-tests and ANOVA as part of the Evaluate step of the process and reported in the summary statement in Step 5.
Eta-square is probably not of much use in the single sample t-test.
It is calculated by

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