This story was originally published on HackerNoon at: https://hackernoon.com/instrument-variables-and-ab-testing-part-1. This article explores the Mathematical details of least squares estimator in an unbiased and biased settings due to model specification errors. Check more stories related to data-science at: https://hackernoon.com/c/data-science. You can also check exclusive content about #linear-regression, #ab-testing, #casual-analysis, #bias, #multicollinearity, #confounding, #instrument-variables, #instrument-variables-math, and more. This story was written by: @varunnakra1. Learn more about this writer by checking @varunnakra1's about page, and for more stories, please visit hackernoon.com. This article explores the Mathematical details of least squares estimator in an unbiased and biased settings due to model specification errors.