How to interpret the transformation guide for fitting linear regression? These two figures give the plot of x vs y, and how to transform either or both x or y variables to fit a linear regression. How to interpret the following transformation schemes? Especially, how to interpret the last column “Linear Form”  and “Linearizable Function” given the transformation? note: Figure a and b are the first two graphs. c and d are the second two below a and b and so on.


 A: Regression via OLS can only be applied if the model equation is linear in the parameters. This means that all parameters $\beta_i$ that are to be estimated need to be either added, subtracted or multiplied to the regressors $X_i$. They cannot be a power or be in a logarithm for example (any non-linear operation). This is so because non-linear parameters cannot be written in matrix form and hence cannot be minimized algebraically yielding a unique solution via minimizing a squared loss (numerical optimizations are possible). The restriction does not hold for the regressors themselves, hence "linear regression" does not mean you can't have for example $X^2$, but that you can't have for example $2^\beta$.
In your table, the "linearizable functions" are functions non-linear in the parameters (note how the $y$s are not linear in the $\beta$s). However, by transforming $y$ using the function in the column "transformation", the function can be made linear in the parameters and thus estimate-able via OLS. The form shown in the last column "linear form" is the form you can do OLS with.

Take for example the first row:

*

*$y = \beta_0 x^{\beta_1}$. Here $\beta_0$ is linear, $\beta_1$ is in the exponent.


*Take the log of 1. to get $\log(y)=\log(\beta_0)+\beta_1\log(x)$. Here, $\beta_1$ has been linearized, so no more problems there. However, $\beta_0$ is inside the $\log$ which is not linear. However, when we do the LS regression, we will get $\log(\beta_0)$ as an estimate which we can then simply exponentiate to get $\beta_0$. Since no regressor $x$ is attached to (e.g. multiplied by) $\beta_0$, this is no problem.


*Define $\log(y)=y'$ and $log(x)=x'$. Now to do OLS, simply use $\log(y)$ as a response and $\log(x)$ as a regressor (instead of the original $y$ and $x$).
