Introduced on day 5
Confounder
A common cause of the treatment and the outcome, which creates an association between them even without a causal effect.
In detail
In airline pricing, demand (season, holidays, booking pace) raises both the fare that revenue management sets and the bookings, so fares and bookings correlate positively although higher fares reduce bookings. Randomization or adjusting for every confounder removes the bias; adjusting for only the recorded ones may not.