PSYC2009 Study Guide - Quiz Guide: Conditional Probability, Deductive Reasoning, Sensitivity And Specificity
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 PA and B aay so e don’t count those eents tice
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 hae a disease, een though you don’t:
o
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