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Chapter 5

# Chapter 5- Jan. 22nd.docx

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University of Guelph

Psychology

PSYC 1010

c

Fall

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nd
Chapter 5- Sampling and Probability Jan. 22
Samples
Two main types of samples:
- Random Sample
Every member of the population has an equal chance of being selected
into the study
- Convenience Sample
Uses participants who are readily available
Limits our generalizability (researchers ability to apply findings from one
sample or in one context to other samples or contexts) if it results in a
sample that is not representative of the population
Replication- refers to the duplication of scientific results, ideally in a
different context or with a sample that has different characteristics (do the
study again, if the study produces similar results, although the participants
are drawn at random, then the study if more valid)
Volunteer sample- special kind of convenience sample in which
participants actively choose to participate in a study, participants volunteer
or self-select
Random assignment is frequently used, refers to a method we can use
once we have a sample, whether or not the sample is randomly selected
Random sample is hardly ever used, refers to a method of creating a
sample from a population
Random Assignment
- If a study has two levels of IV, then participants are randomly assigned to one of
two groups
- Although some patterns may be seemingly non random, they are still considered
to be random
- We still run the risk of an accidentally biased sample giving us bad information
- Researchers use two main tools to make up for a lack of random selection (when
possible)
- With random assignment, every participant has an equal chance of being assigned
to any level of the independent variable
- With replication, a study is repeated, ideally with different participants or in a
different context, to see whether the results are consistent
Probability
- The likelihood that a certain outcome will occur out of all possible outcomes
- The actual likelihood of a given outcome in the long run
- Ex. Likelihood of getting heads when flipping a coin
- When you hear PROBABILITY think PROPORTION.
- When you hear PROPORTION think AREA (or percent)
Expected Relative-Frequency Probability
- Likelihood of an event occurring based on the actual outcome of many, many
trials
- Only works in the long run- law of large numbers - Trial- each occasion that a given procedure is carried out
- Outcome- the result of a trial
- Success- the outcome for which we’re trying to determine the probability
- Personal Probability
The likelihood of an event occurring based on an individual’s opinion or
judgment (subjective probability)
Ex. There’s a 75% chance I’ll finish my paper- our best guess, personal
estimated
- To calculate probability:
Probability= successes / trials
Determine the total number of trials
Determine the number of these trials that are conside

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