1. 13.1 Sampling and Bias This is a very important topic because it will help you understand when a survey/study is done well. If it is done correctly (unbiased), then you can trust the information. If it is done incorrectly (biased), the information gained may not be credible.

2. EXAMPLE: There are lots of ways to misuse statistics. One way is perseverance. If at first you don’t succeed, try, try again. Suppose you want to claim in a TV commercial that 4 out 5 dentists recommend your toothpaste. You ask 5 dentists, but only 1 of them recommends your brand. So, forget you ever asked them! Ask another 5 dentists! This time, 2 of them recommend your brand. Forget them! Ask another 5! Keep trying until you get lucky and 4 of 5 recommend your brand. Then show your TV commercial.

3. Whatever you do, do not talk about the 13,925 dentists you had to survey before you got lucky, and don’t mention that only 8% of them recommend your brand.

4. Population Sample unbiasedbiased simplestratifiedsystematic conveniencevoluntary Copy this down

5. EXAMPLE 1 I’m a business owner trying to see if there is a strong desire to have a drive-in theater here in Ahwatukee. Population: all of Ahwatukee and surrounding areas (I wish I could survey every one of these people but I can’t. Census = entire population Sample: a subgroup of the population that represents the entire population.

6. 3 Unbiased ways to get my sample 1) Simple random: I could open the phone book of Ahwatukee and the surrounding areas and randomly choose 100 numbers out of the phone book to call and survey. (Each person in the population has a chance of being selected.) 2) stratified: I pick ten random numbers to call within each city block. (The large group is first divided into smaller groups) (Each person in the population has a chance of being selected.)

7. 3) Systematic: I call 1 person on each tenth page. (sample selected according to a specified time or interval)

8. 2 Biased ways to obtain a sample 1) Convenience: I ask 10 of my friends what they think about opening a drive-in. (I choose an easy sample.) (Not everyone has a chance of being selected) 2) Voluntary: I mail 100 surveys out and get info from the people that sent it back. (Any time the people have a choice of being involved, it will have voluntary bias.)