1. Why Sample in the Census?

2. What they planned to do Normal effort Special effort
Census divides the US into 7,000,000 blocks
Tries to get everyone in every block
Special effort
Take 10,000 blocks (randomly selected)
Do completely independent count
Cross check the lists

3. Why? Capture-recapture designs
Have 20 researchers (covering different regions) catch and tag fish in a lake. Say they get 40 in all.
Say the next day they fish again.
They catch 50 fish.
8 have tags (8 of the 40 original tags).
How many fish are there in the lake?
50/(8/40) = 250
(second capture sample)/(recapture ratio)