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Week3_Notes - Psych 100A Week 3 Discussion Notes Dawn Chen...

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Psych 100A Week 3 Discussion Notes Dawn Chen October 16, 2009 Probability Terms Probability : How likely an event is to occur Outcome : One possible way something could happen; a result of a random experiment Sample space : The set of all possible outcomes (e.g., the sample space for flipping a coin twice is {HH, HT, TH, TT}) Event : A subset of the sample space (e.g., the event of getting at least one heads when tossing a coin twice is {HH, HT, TH}) Probability distribution : Divides the sample space into non-overlapping portions and assigns a probability to every portion Notation ( ) P A : Probability of the event A ( ) P A : Probability of A not occurring ( ) P A B : Probability of A or B occurring ( ) P A B : Probability of A and B occurring Properties 0 ( ) 1 P A Every probability distribution sums to 1. Formulas If every outcome is equally likely, # of outcomes in ( ) total # of outcomes A P A = If A and B are mutually exclusive, ( ) ( ) ( ) P A B P A P B = + If A and B are not mutually exclusive, ( ) ( ) ( ) ( ) P A B P A P B P A B = + - The Normal Distribution The normal distribution is a theoretical probability distribution that is useful because it approximates many distributions of real-world measures that are determined by a large number of factors, such as IQ. The normal distribution is symmetric , unimodal , and continuous . It extends infinitely to the left and right, always approaching 0 but never quite reaching 0.
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