I am confusing with the wording here. I was reading a book on linear regression. "The primary concern for linear models is that they display linearity in the parameters. Therefore, when we refer to a linear regression model we generally assume that the equation is linear in the parameters; it may or may not be linear in the variables"
Can anyone explain that last sentence to me? As it seems a bit odd to say so.. Like: y=a+b*x^2 In my understanding, this is linear in parameters, not linear in the variables, so that makes it a linear model? It feels like it should be the other way round, can someone please explain? Thanks.