# Practical power analysis on linear model

Power analysis for linear model test asks for an analytical expression of the power of a linear model. The answer claims that

Note that this depends on the correlation between $$X$$ and $$T$$, $$r_{TX}$$.

In practice, when running the power analysis of an experiment, prior to the experiment, you don't really know this (the treatment has not been assigned yet, you don't have data labelled on treatment and control). At most, you have historical data with the covariate X, the outcome Y and a way to randomize the treatment.

How do we calculate $$r_{TX}$$ in practice? Do we simulate a specific randomization and calculate power with that randomization? Do we simulate many and average the power? Is there a standard way of doing this?

Also, how do we estimate $$\sigma$$ and $$\sigma_T$$ in practice?

• I don't think there is a standard way. I always recommend going with a "reasonable" simulation, based on knowledge about potential correlations. This has the advantage of allowing sensitivity analyses - if your power depends strongly on the specific value of this correlation, this is a valuable piece of information. Mar 27 at 9:42
• How are you going to assign treatment to subjects? If it's at random, why would you expect the covariate X and the treatment assignment to be correlated? Mar 27 at 10:01
• The idea is to replicate the assignment that is going to be done in the experiment. I understand that in some cases it would be 0, but other cases (cross-over trials, clustered designs, stratified designs) this could be non-0. Is your suggestion to assume that the value of r_tx is 0? Mar 27 at 11:24
• I think that, in general, assuming that it is 0 will give you over-confident results, as when you analyze the data of the experiment it may be close to 0, but non-0 Mar 27 at 11:25
• I understood this question to be about a particular experiment. It seems rather it's more of a theoretical question since you'd like to consider so many different experimental designs (cross-over trials, clustered designs, stratified designs). Thank you for the clarification. Mar 27 at 11:41

Although analytical power formulas provide general guidance, the large level of uncertainty about the values of the parameters in the formula means that it's wise to evaluate a range of possibilities in study design. See this page, for example, about the difficulty simply in evaluating the value of a linear-regression error $$\sigma^2$$ from a limited data set. You want to be covered in case your initial estimate is incorrect.