# What is the difference between these two, and are they both actually the population linear regression model?

I am trying to understand the difference between these two stated population linear regression models. My professor's notes state that, if we have predictors $$X_1, \dots, X_p$$, then the population linear regression model is $$Y = \beta_0 + \beta_1 X_1 + \beta_2 X_2 + \dots \beta_p X_p + \epsilon$$. But I have also seen the population linear regression model stated as $$Y_i = \beta_0 + \beta_1 X_i + \epsilon_i$$. What is the difference between these two, and are they both actually the population linear regression model?

## EDIT

If you Google "linear regression population model," you'll find these images:

• I don't think either of those make sense for the term, but especially that second equation makes absolutely no sense Nov 3, 2021 at 16:07
• Ah I think I understand the confusion I will post in an answer in a few min. Nov 3, 2021 at 16:09
• @bdeonovic If you Google "linear regression population model," you'll find these images (see my edit). Nov 3, 2021 at 16:10
• Surprisingly people on the internet can be wrong, or glossing over details Nov 3, 2021 at 16:20
• The slides you posted describe "simple" linear regression (ie linear regression with just one covariate) so they have dropped the index for covariate number Nov 3, 2021 at 16:21

The linear regression model is

$$y_i = \beta_0 + \beta_1 x_{1i} + \cdots + \beta_p x_{pi} + \epsilon_i$$

for $$i=1,\ldots,n$$ and $$\epsilon_i \sim N(0,\sigma^2)$$ and independent

Here $$y_i$$ is the outcome (or dependent variable) for unit $$i$$ (e.g. person $$i$$). Sometimes this indexed is dropped and simply stated as

$$y = \beta_0 + \beta_1 x_{1} + \cdots + \beta_p x_{p} + \epsilon$$

"simple" linear regression is linear regression with just one covariate in which case you have $$y_i = \beta_0 + \beta_1 x_i + \epsilon_i$$ Note we can drop one of the indexes on the covariate $$x$$ to indicate which of our covariates it is, since there is only 1.

The "population" linear regression isn't a well defined term. Probably most people would assume this means the population mean in the above linear regression which would simply be $$\mu = \beta_0 + \beta_1 x_{1} + \cdots + \beta_p x_{p}$$

Additionally, linear regression can be expressed more compactly in matrix notation as

$$\mathbf{y} = \mathbf{X}\mathbf{\beta} + \mathbf{\epsilon}$$

Where $$\mathbf{y}$$ is the $$n\times 1$$ vector $$\mathbf{y}=(y_1,\ldots,y_n)$$, $$\mathbf{X}$$ is the $$n\times p$$ matrix

$$\begin{pmatrix} x_{11} & \cdots & x_{1p}\\ & \ddots & \\ x_{n1} & \cdots & x_{np} \end{pmatrix}$$ and $$\epsilon$$ is the $$n\times 1$$ vector $$\epsilon = (\epsilon_1,\ldots,\epsilon_n)$$

• Thanks for the answer. When my professor says "population" linear regression model, I think they are contrasting it with the estimator model $\hat{y} = \hat{\beta_0} + \hat{\beta_1} x_1 + \dots + \hat{\beta_p} x_p$, since this is covered in the same notes. So the use of the word "population" here would be to differentiate the model from the "realised"/"estimator" model. Makes sense, right? Nov 3, 2021 at 16:27
• that could be what they meant, but that is not typically how the term "population" is used. It typically refers to the mean $\mu = \beta_0 + \beta_1x_1 + \cdots + \beta_p x_p$. $\epsilon$ refers to the individual variation away from this mean. That is why this value $\mu$ is typically referred to as the "population" mean Nov 3, 2021 at 16:42
• Ok, thank you for taking the time to clarify this. Nov 3, 2021 at 16:43