1) "Simple linear regression model: Y = β0 + β1X + ε E(Y) = β0 + β1X A linear model means that it is linear in β's, and not necessarily a linear function of X. The independent variable X could be W2 or ln(W), and so on, for some other independent variable W." I have some trouble understanding the last line. I was told that a SIMPLE linear regression model is always a straight line model, it is a least-square LINE of best fit. But if X=W2, then we have E(Y) = β0 + β1W2 which is not a straight line...how come?? Is this allowed? 2) "A SIMPLE linear regression is a linear regression in which there is only ONE independent variable." Now is the following a simple linear regression or a multiple linear regression? Y = β0 + β1X + β2X2 + ε It has only one independent variable X, so is it simple linear regression? But this just looks a bit funny to me... 3) "A linear regression model is of the form: Y = β0 + β1X1 + β2X2 + ... + βkXk + ε If there is more than one independent variable, then the model is called a MULTIPLE linear regression model." This idea doesn't seem too clear to me. What can the Xi's be? What are some actual examples of mutliple linear model? Does a linear model always have to be a straight line or a plane? Thanks for explaining!