Exercise lesson 130
Hello, I’m struggling to understand the formula used in the exercise to calculate the churn rate:
SELECT
100 - 100 * COUNT(DISTINCT(b.visitor_id)) / COUNT(DISTINCT(h.visitor_id))::float AS churn_rate
FROM web_analytics.pageviews h
I don’t get the 100-….why cannot we leave it as:
100 * COUNT(DISTINCT(b.visitor_id)) / COUNT(DISTINCT(h.visitor_id))::float
Many thanks!
REPLIES
@Ale.1, that’s a great question, it’s super important to distinguish conversion rate from the churn rate 
Conversion rate
Example: 10 users visited the checkout page, 2 of them purchased. The conversion rate (in that case, conversion to purchase) is 20%.
Churn rate
The same example: 10 users visited the checkout page, 2 of them purchased. It means that 8 didn’t, or they churned. Churn rate is 80%.
As you can see, churn rate and conversion rate should add up to 100%.
That’s why in the lesson we calculate the conversion rate (visitors who went to a book page from the homepage):
100 * COUNT(DISTINCT(b.visitor_id)) / COUNT(DISTINCT(h.visitor_id))::float
and then subtracting it from 100 to get a churn rate (visitors who didn’t go to a book page).
That’s it 