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spcs:summer2014:day_10

# SPCS - Day 10

Free Seating Friday!

I'm doing one-on-one meetings with each of you on Monday!

## Assignment

Math Probability Problem (Optional Challenge)

There is an urn containing three balls. Each ball is either red or blue, and two balls have the same color. So, there are two possibities: either two balls are red and one is blue (the “majority-red case”) or two balls are blue and one is red (the “majority-blue” case). The probability of both cases is the same, i.e., 50%. John is allowed to draw a random ball from the urn, look at it, and put it back. Then, he has to make a guess as to whether he thinks the urn is majority-blue or majority-red. Let’s say that John has drawn a red ball. It seems natural that John should guess that the urn is probably majority-red. We now ask you to confirm this is a good guess, using Bayes’ rule.

Let “MajRed” be the event that the urn is majority-red. So, “not MajRed” is the event that the urn is not majorityred, which means that the urn is majority-blue. Let “RedBall” be the event that John picked a red ball. We are interested in the probability P(MajRed|RedBall) that the urn is majority-red given that John has picked a red ball.

• (a) What is the prior probability P(MajRed) according to the above description?
• (b) What is the conditional probability P(RedBall | MajRed)?
• © What is the conditional probability P(RedBall | not MajRed)?
• (d) Finally, using Bayes’ rule and other rules, what is the conditional probability P(MajRed | RedBall)?

John decided to declare that he thinks the urn is majority-red. Next, it’s Mary’s turn to pick a random ball from the rn, look at it, and put it back. She picks a blue ball. Now, she is in the same situation as John: she has to make a guess about the urn based on what she has seen. But, she also knows that John has seen a red ball (because otherwise he would not have said that he thinks the urn is majority red).

Mary is not sure what to think. On the one hand, she saw a blue ball, and therefore the urn is most likely majorityblue. On the other hand, she knows that John has picked a red ball, which suggests that the urn is majority-red.

We can solve this problem again using Bayes’ rule. Let “RedThenBlue” be the event that John picked a red ball, and then Mary picked a blue ball (which is what happened).

• (e) What is the conditional probability P(MajRed | RedThenBlue)?

Hint: this can be computed using the same techniques as in (a)-(d).

## Blog

• Projects (as usual)

## Arduino

• You know what to do!
• Take pictures and blog it!

Have fun! Great work, class!