An earlier version of this text included a math miscalculation in the Engagement and Analytics chapter. The sentence has since been corrected to read:

“Did 1,000 people attend the festival (if so, you sold hats to 75% of them, and that’s excellent!), or did 50,000 people attend (if so, you sold hats to only 1.5% of them, and that’s not so excellent, assuming everyone at the festival had a head)?”