This is a quick overview of how to use **asreg** command for running regressions by groups. To know about the installation and other details of asreg, please click here.

## Regressions by groups

A group is a subset of data that has a common identifier. Examples of groups can include families, industries, countries, regions, months, years, portfolios, etc. If a linear regression needs to be estimated separately for each group, **asreg** is the best option. **asreg** is blinking fast and writes all regression outputs to the data in memory as separate variables. This eliminates the need for writing the results to a separate file which is usually then merged back to the data for any further calculations. **asreg** eliminates all these steps and provides convenient and clean solution to by-group regression and rolling regressions. IF not installed, **asreg** can be installed from ssc with this line of code:

ssc install asreg

## An Example

We shall use the *grunfeld* data set in our example. Let’s download it first:

webuse grunfeld

Assume that we want to estimate a regression by year (in this example, year is our grouping variable) where the dependent variable is *invest* and independent variables are *mvalue* and *kstock*. Just like *regress* command, **asreg** uses the first variable as dependent variable and rest of the variables as independent variables. Using the *grunfeld* data, **asreg** command for estimating yearly regression would be:

bysort year: asreg invest mvalue kstock

Please note that the grouping variable *year* comes after the *bysort* prefix which is followed by full colon. Similarly, if we want to estimate the above regression for each company, our grouping variable would be company that will come after the *bysort* prefix:

bys company: asreg invest mvalue kstock

## Standard errors and fitted values

If standard errors are needed for each explanatory variable, we shall just add the otpion ** se**. Similarly, we can add option

**if fitted values are required.**

*fit*bys company: asreg invest mvalue kstock se

bys company: asreg invest mvalue kstock se fit