The article "Mostly Harmless Econometrics: An Empiricist's Companion" discusses how basic econometric tools like linear regression and instrumental variables can help answer important questions about cause and effect in social science. The researchers show that fancy techniques are often unnecessary, and simpler methods can be more reliable. They also introduce new methods like regression-discontinuity designs and quantile regression. Overall, the article emphasizes the practical use of econometric tools in various social science fields.