# Eloquent Javascript - correlation and coefficient

Currently I'm reading Eloquent Javascript book and at chapter 4. This topic entitled 'The lycanthrope’s log ' is very confusing to me. How important is correlation and coefficient in real world programming?

I'm trying to understand the formula used to find correlation. I've far forgotten maths since I left my high school. Could, someone who've read this book, explain to me in detail, please? Below is the picture I'm wondering about..

• When you say "correlation and coefficient", you appear to be talking about two things -- what coefficient do you mean, besides the correlation? – Glen_b -Reinstate Monica Jun 10 '16 at 7:29

The formula is actually pretty straight forward. Consider the table from your question.

The table states that there were a total 90 observations

Out of all 90 observations:

1. 76 times, no pizzas or squirrel
2. 9 times, pizzas but no squirrel
3. 4 times, squirrel but no pizzas
4. 1 time, both squirrel and pizza

So we want to find out how Pizzas & Squirrels are related:

Consider the equation along with your table now,

(since squirrel is on the left)

### n.1 refers to the sum of all measurements where we have pizza

Now that we know what these values are, let's plug them in to our formula:

$$\mathit{correlation} = \rho = \frac{ 1 \times 76 - 4 \times 9}{\sqrt{5 \times 85 \times 10 \times 80}}$$

• Well, tbh I had to research the thing first :P – atefth Jun 10 '16 at 7:13
• It might be worth noting that if you code the binary variables for "no-squirrel/squirrel" and "no-pizza/pizza" as (0,1) (or indeed any other two distinct values where the "no" part is either lower for both or higher for both) then this is actually the same as the ordinary correlation coefficient. – Glen_b -Reinstate Monica Jun 10 '16 at 7:43