PSYC1010 - UNIT 3 Notes

PSYC1010 Unit 3: Probability – Part 1

Probability
 “...is the likelihood that any one event will occur, given all the possible outcomes”.
 “p” = probability (ratio or decimal)
 Assigning a probability value to an inference
 Example
o Flipping a coin, rolling a dice
Probability as a Mathematical System
 The sample space is a set of points (data)
 Persons, businesses, cities, schools
 What are events?
o Observable occurrences
 A heads on a coin flip
 S
o Sample space
o In red
 Event
o A single element in the sample space
o 5 stars, 3 triangles, 3 circles
 11 total events
o If you don’t know events, you cannot calculate probability
Probability Statement
 A statement of the relative frequency of an event associated with the sample space
 Example
o English alphabet
 What is the sample space?
 26 letters, A to Z inclusive
 What is the probability of event “A” occurring?
 P(A) = probability of event A
 Definition
o The probability of event A, P(A), is the ratio of the number of sample points that
are examples of A to the total FINITE number of sample points in the sample
space, assuming all sample points are equally likely
Probability Example
 What is the probability of rolling a 3 (facing up) on a single dice?
o What is the sample space?
 6 possible sample points, equally likely, 1 to 6 inclusive
o What is the probability of event A?
 Event A = rolling a 3 up
o P(A) = probability of event A
 P(3) = probability of rolling a 3
o P(A) = # of examples of A / Total number of sample points
 P(3) = 1/6
 = 0.167