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PSY031 Spring 08
Answers Lab #6
Page 1 of 4
Lab Partners:
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I.
Fun with Probability.
1)
A bag contains red and blue marbles.
One marble is drawn at random from the bag.
Each
marble from the bag has an equal chance to be drawn.
If it is red, you win $1.
If it is blue, you
win nothing.
You can chose between two bags:
Bag A:
Contains 3 red marbles and 2 blue marbles.
Bag B:
Contains 60 red marbles and 40 blue marbles.
Which bag offers the better chance at winning?
Why?
Bag A:
3
0.60
5
chance of winning
=
Bag B:
60
0.60
100
chance of winning
=
So if you’re drawing one marble from random, then both bags have equal probability when
it comes from winning.
It really doesn’t matter what bag you pull from.
2)
A coin is tossed more than 100 times, landing heads 53 times.
However, the last seven tosses
are all heads.
We’re assuming it is a fair coin.
True or False:
The chance that the next toss will
be heads is somewhat less than 50%.
Explain.
False.
The chance that the next toss will be heads is p=0.50.
Why?
Because each coin toss
is independent of the other.
3)
Kevin Garnett is fouled in the act of shooting with 0.01 seconds remaining in the game.
He
gets two free throws.
The Celtics are trailing by one point.
Garnett makes 78% of his free
throws.
Assuming free throws are independent events, what is the probability that he will make
one of the two free throws to send the game into overtime?
What is the probability that he will
make both to win the game?
What is the probability that he will miss both and lose the game?
Tie:
[P(hit) and P(miss)] OR [P(miss) and P(hit)]
= (0.78)(0.22) + (0.22)(0.78) = 0.3432
Win:
P(hit) and P(hit) = .78*.78 = 0.6084
Lose:
P(miss) and P(miss) = .22*.22 = 0.0484
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PSY031 Spring 08
Answers Lab #6
Page 2 of 4
II.
The Binomial Distribution – Lie Detection
The data from class are in the file “LieDetections” found under COURSE
DOCUMENTS>LAB INFORMATION>LAB5. The experiment involved three
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 Spring '08
 Dicorcia

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