Should we learn this formula by heart? I’m overdosing on formulas, if i can let go of one, I will. As per LOS: “distinguish between and interpret the R2 and adjusted R2 in multiple regression;”
I have (I think) an easier way to remember the “formula” for Adjusted R-squared.
MSE (Mean Squared Error) = SSE/(n-k-1) and VAR(Y) = SST/(n-1)… where SSE is the sum of squared errors and SST(Y) is the total sum of squares for Y (total variation in Y)… These are just variances (MSE and VAR(Y)), so this should seem less daunting.
Adjusted R^{-squared} = 1 - [MSE/VAR(Y)] … try it out with a couple of practice problems— after accounting for the degrees of freedom (which is partly the point of adjusting R^{-squared} ), the explained variance is total variance of Y minus the unexplained variance of Y (SER squared, MSE). To calculate adjusted R^{-squared}, simply put this difference over the total variance of Y— this simplifies to 1 - [MSE/VAR(Y)].
Regular R-squared is 1 – [SSE/SST(Y)].
Realistically, you’re not dealing with anything new, exactly. If you remember it in terms of the variances, I don’t think it will be as difficult to remember or work with going forward.
For the LOS, I think it’s safe to assume they want you familiar with some of the distinctions between the two forms and to know when one is appropriate versus the other.
Well, I tried haha… Honestly, I can’t see them wasting their time asking you to calculate adjusted R-squared, but, then again, I could see someone saying “a calculation would imply a basic understanding” or something like that. Either way, we’re not going to know everything. I think this LOS is pretty low-yield-- aside from knowing why you might make an adjustment to R-squared (just my opinion, though).
It’s from LOS 9i- calculate and interpret a confidence interval for the predicted value of the dependent variable. I believe we are suppose to know it.
In the real world, I agree, but being familiar with the calculation is certainly helpful when you run into issues or are evaluating your model. However, this doesn’t seem to be the best rule of thumb. If they ask you to calculate and interpret the global f-test for total model utility, I would say you don’t understand the basic idea of the f-statistic for the test if you’re unable to calculate it (same goes for r-squared, t-tests, etc.). If you truly have intuition about these ideas, the calculation should be easier to remember.