Wednesday, October 17

ANNOUNCEMENT: A NEW FIELD OF STUDY! The Stanford Journal Of Body Part Estimating Ability Inaugural Special Issue: Hands. Next: Toes. Volume 1, No. 1. October, 2012

We now move from the world of description and prediction to the world of hypothesis testing and decision-making using probability…

We now turn to the world of urns…

Population Sample You take a sample, and use a probability model to make a decision about its noteworthiness.

“I am not a crook.”

High Stakes Coin Flip

Could your professor be a crook? High Stakes Coin Flip Could your professor be a crook?

Could your professor be a crook? High Stakes Coin Flip Could your professor be a crook? Let’s do an experiment.

The Coin Flip Experiment Question: Could the professor be a crook? Let’s do an experiment. Make assumptions about the professor. Determine sampling frame. Set up hypotheses based on assumptions. Collect data. Analyze data. Make decision whether he is or is not a crook.

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook.

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook. Step 3. Determine the risk that you are willing to take in making an error of false slander,  (alpha), often at .05

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook. Step 3. Determine the risk that you are willing to take in making an error of false slander,  (alpha), often at .05 Step 4. Decide on a sample, e.g., 6 flips.

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook. Step 3. Determine the risk that you are willing to take in making an error of false slander,  (alpha), often at .05 Step 4. Decide on a sample, e.g., 6 flips. Step 5. Gather data.

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook. Step 3. Determine the risk that you are willing to take in making an error of false slander,  (alpha), often at .05 Step 4. Decide on a sample, e.g., 6 flips. Step 5. Gather data. Step 6. Decide whether the data is more or less probable than  . E.g., the probability of 6 consecutive wins based on the assumption in Step 1 is .016. (.5 x .5 x .5 x .5 x .5 x .5 = .016)

Some Steps in Hypothesis Testing Step 1. Assume that the professor is fair, i.e., that P(Win) = .5 Step 2. Set up hypotheses: H0: He is not a crook. H1: He is a crook. Step 3. Determine the risk that you are willing to take in making an error of false slander,  (alpha), often at .05 Step 4. Decide on a sample, e.g., 6 flips. Step 5. Gather data. Step 6. Decide whether the data is more or less probable than  . E.g., the probability of 6 consecutive wins based on the assumption in Step 1 is .016. (.5 x .5 x .5 x .5 x .5 x .5 = .016) Step 7. Based on this evidence, determine if the assumption that Kenji is fair (not a crook) should be rejected or not.

What’s the probability of rolling a dice and getting 6?

Rolling a six (6) Six possible values (1,2,3,4,5,6) = 1/6 = .17

What’s the probability of rolling a dice and getting an even number?

Rolling an even (2, 4, 6) Six possible values (1,2,3,4,5,6) = 3/6 = .50

What the probability that your first (or next) child will be a girl?

What is the probability of flipping 8 heads in a row?

What is the probability of flipping 8 heads in a row? .5 x .5 x .5 x .5 x .5 x .5 x .5 x .5 or .58 = .004

What is the probability of flipping 8 heads in a row? .5 x .5 x .5 x .5 x .5 x .5 x .5 x .5 or .58 = .004 Formalized as: The probability that A, which has probability P(A), will occur r times in r independent trials is: P(A)r

So, you decide to conduct a case study of 3 teachers, sampling randomly from a school district where 85% of the teacher are women. You end up with 3 male teachers. What do you conclude? P(males) three times = P(males)3 = .153 = .003

So, you decide to conduct a case study of 3 teachers, sampling randomly from a school district where 85% of the teacher are women. You end up with 3 male teachers. What do you conclude? P(males) three times = P(males)3 = .153 = .003 If you had ended up with 3 female teachers, would you have been surprised?

What do you notice about this distribution? Number of Heads Probability 0 1/64 =.016 1 6/64 =.094 2 15/64 =.234 3 20/64 =.312 4 15/64 =.234 5 6/64 =.094 6 1/64 =.016 ___________ 64/64 =1.00

What do you notice about this distribution? Number of Heads Probability 0 1/64 =.016 1 6/64 =.094 2 15/64 =.234 3 20/64 =.312 4 15/64 =.234 5 6/64 =.094 6 1/64 =.016 ___________ 64/64 =1.00 Unimodal

What do you notice about this distribution? Number of Heads Probability 0 1/64 =.016 1 6/64 =.094 2 15/64 =.234 3 20/64 =.312 4 15/64 =.234 5 6/64 =.094 6 1/64 =.016 ___________ 64/64 =1.00 Symmetrical

What do you notice about this distribution? Number of Heads Probability 0 1/64 =.016 1 6/64 =.094 2 15/64 =.234 3 20/64 =.312 4 15/64 =.234 5 6/64 =.094 6 1/64 =.016 ___________ 64/64 =1.00 Two tails

GAUSS, Carl Friedrich 1777-1855 http://www.york.ac.uk/depts/maths/histstat/people/

f(X) = Where  = 3.1416 and e = 2.7183 1  2 e-(X - ) / 2  2

Normal Distribution Unimodal Symmetrical 34.13% of area under curve is between µ and +1  34.13% of area under curve is between µ and -1  68.26% of area under curve is within 1  of µ. 95.44% of area under curve is within 2  of µ.

Some Problems If z = 1, what % of the normal curve lies above it? Below it? If z = -1.7, what % of the normal curve lies below it? What % of the curve lies between z = -.75 and z = .75? What is the z-score such that only 5% of the curve lies above it? In the SAT with µ=500 and =100, what % of the population do you expect to score above 600? Above 750?