# Linear regression by hand – The Data Science Gazette

… most companies would be taking a huge step forward if they got somebody who knows how to do linear regression.

When you have predictors of a scalar-valued outcome with observations indexed by and residuals denoted , a model of the form

or (equivalently) in matrix notation

is best estimated using ordinary least squares, the workhorse of linear regression. The underlying math is a fair bit of matrix algebra which, when all is said and done, returns

This was the one equation my graduate school program director urged every student to know by heart.

X prime X inverse X prime y, “prime” meaning transpose, yields OLS estimates of linear regression coefficients. On the left-hand side, the hat symbol denotes an estimate from a sample as opposed to a true value in the population.

We can walk through a real-data example using `mtcars`, an automobile-themed dataset built into R. Regressing fuel economy (mpg) on weight (wt) and number of cylinders (cyl),

by

```summary(lm(mpg ~ wt + cyl, data=mtcars))
```

will give you

```            Estimate Std. Error t value Pr(>|t|)
(Intercept)  39.6863     1.7150  23.141  < 2e-16 ***
wt           -3.1910     0.7569  -4.216 0.000222 ***
cyl          -1.5078     0.4147  -3.636 0.001064 **
```

wherein the estimates (first column of numbers) from top to bottom correspond to

Per the foregoing all-important equation, only two objects are necessary to compute the estimates manually: (1) the matrix and (2) the vector . Both are easy to extract from `mtcars`.

```## getting `X` the easy but obfuscated way
X = model.matrix(mpg ~ wt + cyl, data=mtcars)

## getting `X` a more transparent way:
## grab only the `wt` and `cyl` columns from mtcars
## and prepend a column of ones to represent the intercept
X = cbind(1, mtcars[, c("wt", "cyl")])
X = as.matrix(X) # transform data.frame to matrix type

## getting `y`
y = mtcars[, "mpg"]
```

Given and , all that’s left are matrix operations. Mathematically they are described in this tutorial from Harvey Mudd College. Computationally, `t()` transposes, `solve()` inverts, and `%*%` multiplies matrices.

Now for the moment of truth—X prime X inverse X prime y.

```solve(t(X) %*% X) %*% t(X) %*% y
```
```         [,1]
1   39.686261
wt  -3.190972
cyl -1.507795
```

This formula doesn’t get us p-values but who needs those anyway.



OLS is BLUE—the

best linear unbiased estimator

—under certain assumptions that are very important but beyond the scope of this post.