PSYC2009 Study Guide - Quiz Guide: Conditional Probability, Deductive Reasoning, Sensitivity And Specificity

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14 Jun 2018
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Probability
Priori probability is a probability that is derived purely by deductive reasoning.
Will always take a value between 0 and 1. can be expressed as a fraction, decimal, or a
percentage
Basic probability
The probability of even A occurring can be determined by the ratio of the frequency of A's
occurrence over the total sample size
o    
   
Compound event
Compound events happen when more than one event is occurring, with these events joined by
words like "and" and "or"
The addition rule can be used to determine the probability of compound events
o      
o We take PA and B aay so e don’t count those eents tice
Conditional probability
Used to determine the probability of some event occurring given some other event already
occurred
Dependant on the order of event occurrence
o   

o Where P(A|B) can be read as "the probability of A given that B HAS happened
Terms:
o If P(A|B) = P(A) and then A and B are independent variables. This means one isn't
predictive of the other
o If the two events are independent then P(A and B) = P(A)P(B). This is the multiplication
rule
o If P(A and B) = 0 then the events are mutually exclusive. This means they cannot co-
occur
Use of tables in probability
Sport
No sport
Total
Job
5
5
10
No job
10
10
20
Total
15
15
30
Bolded and blue values given in question, need to define other values (can do by addition,
etc.)
Can use this to work out probabilities:
o  
  
o  
  
o   
  
o   
 

 
  
o 

 
 
Probabilities in a medical setting
False positive: results say you hae a disease, een though you don’t:
o  
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