What variable can affect the number of books sold by a door to door salesperson?

Variables that can affect the number of books sold by a door to door salesperson could include:

1. whether another salesperson got there first
2. how many people were home
3. how many people answered their doors
4. encountering mean guard dogs
5. how many people liked to read
6. the cost of one book
7. how many people could afford books for self or for family
8. whether pay day just occurred, or not
9. how many bills the person needed to pay like rent and utilities
10. interest in the books being sold
11. whether the salesperson worked the route or was sick
12. whether the salesperson's car broke down
13. being in an accident
14. the weather on work days
15. major calamities, such as a tornado struck that neighborhood