In this post we’ll use the system dataset auto.
sysuse auto, clear
To estimate the model we use the
regress command in the command window. The
regress command follows the general format of
regress dv iv, options. Type
help regress or visit the online help for
regress for a description of the options available for regress. For example the regression of price on mpg is estimated as follows:
regress price mpg
The output includes:
- The ANOVA source table
- Descriptive statistics and effect sizes
- Coefficients, hypothesis tests, and confidence intervals
Suppose we would like Stata to report standardized coefficients. To get standardized coefficients we add the
beta option to our command.
regress price mpg, beta
Visualizing Regression Lines
We can visualize the relationship between two variables with a scatterplot. Stata’s graphics provide several useful commands for including regression lines on a scatterplot. We’ll discuss the
To produce a scatterplot between price (y-axis) and mpg (x-axis), we use the
graph twoway scatter command.
graph twoway scatter price mpg
Now let’s add the regression line to the plot. The
lfit graph command allows us to do this (
lfit stands for linear fit). However, we don’t want the regression line in isolation. We want it on top of the scatterplot. Stata lets you combine twoway graphs in one of two ways: (1) using parentheses or (2) using pipes. To add the regression line with parentheses, we type:
graph twoway (lfit price mpg) (scatter price mpg)
The first set of parentheses is the regression line and the second is the scatterplot. This produces the following plot:
To add the regression line with pipes (this produces an identical plot as above), we type:
graph twoway lfit price mpg || scatter price mpg
It can be nice to include confidence intervals on the plot. To do this we simply change the
lfit command to
lfitci, where the
ci refers to confidence interval.
graph twoway lfitci price mpg || scatter price mpg