Vina Nguyen HSSP – July 20, 2008 1
If we have probability, and conditionalprobability…
We can have independence, and conditionalindependence too
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Definition:P(A B | C) = P(A|C)P(B|C)given C, A and B are independent
Another way to write this:P(A | B C) = P(A|C)
U
U
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We have two coins: blue and red
We choose one of the coins at random (probability = 1/2), and toss it twice
Tosses are independent from each other given a coin
The blue coin lands a head 99% of the time
The red coin lands a head 1% of the time
Events: H1 = 1st toss is a headH2 = 2nd toss is a head
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Bayes’ ruleIndependenceConditional Independence
Things are not always what they seem! But with these tools you can calculate the probabilities accurately
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Where have we seen this?When sample space is finite and made up of equally likely outcomesP(A) = # elements in A
# elements in Ω
But counting can be more challenging…
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Use the tree to visualize stagesStage 1 has n1 possible choices, stage 2 has n2possible choices, etc…
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An experiment with m stages has
n1n2…nm results,
where n1 = # choices in the 1st stage,n2 = # choices in the 2nd stage,
…nm = # choices in the mth stage
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How many ways can we pick k objects out of n distinct objects and arrange them in a sequence?
Restriction: k ≤ n
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Start with n distinct objectsArrange k of these objects into a sequence
# of possible sequences:
= n!(n – k)!
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Start with n distinct objectsPick k to form a set
How is this different from permutations?Order does NOT matterForming a subset, not a sequence
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Pick 4 colors as the universal setHow many 2-color combinations can you create?
Remember that for combinations,
=
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Permutations = 1. Selecting a combination of k items2. Ordering the items
How many ways can you order a combination of k items?
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Start with n distinct objectsArrange k of these objects into a set
# of possible combinations:
= n!k! (n – k)!
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“n” choose “k”
nk
Side note: this is also known as the “binomial coefficient,”used for polynomial expansion of the binomial power [outside of class scope]
( )
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We have a set with n elementsPartition of this set has r subsetsThe ith subset has ni elements
How many ways can we form these subsets from the n elements?
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6 total M&Ms1 of one color2 of one color3 of one color
How many ways can you arrange them in a sequence?
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One perspective6 slots = 3 subsets (size 1, size 2, size 3)Each subset corresponds to a color
At each stage, we calculate the number of ways to form each subset
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Stage #2: Place the second color5 possible slotsNeed to fill 2 slots
# combinations: 52( )
Notice how it does not matter which M&M we place in which slot – this implies order does not matter use combinations
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Start with n-element set (no order)In this set, there are r disjoint subsetsThe ith subset contains ni elements
How many ways can we form the subsets?
n!n1!n2!...nr!
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A class has 4 boys and 12 girls. They are randomly divided into 4 groups of 4. What’s the probability that each group has 1 boy?
Use counting methods (partitions) this time
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Vina Nguyen
MIT OpenCourseWarehttp://ocw.mit.edu
Probability: Random Isn't So RandomSummer 2008
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