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It's All About Variables! 1



It's All About Variables!




Re-written by a global thinker
who was driven nuts by the text book.

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It's All About Variables! 2


Table of Contents
Population versus Sample...................................................................................................................................3
Selecting Samples...............................................................................................................................................3
Simple Random Sample (SRS)......................................................................................................................3
Stratified Random Sampling..........................................................................................................................3
Cluster Sampling............................................................................................................................................3
Systematic Sampling......................................................................................................................................3
Convenience Sampling..................................................................................................................................3
Self-Selected Samples....................................................................................................................................3
Bias..........................................................................................................................................................................4
VARIABLES ­ Things that vary.............................................................................................................................5
Experiment Design ­ Observational or Experimental........................................................................................5
NUISANCE VARIABLES.................................................................................................................................5
CONTROLLING SUBJECT NUISANCE VARIABLES.............................................................................5
CONTROLLING SITUATION NUISANCE VARIABLES.........................................................................6…

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It's All About Variables! 3


Population versus Sample

Populations are all the things we are interested in; they can be all people, cars, cats or toothpicks. We can be
interested in more than one population at the same time.
Samples are a random selection of a proportion of the population…

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It's All About Variables! 4

Bias
Who organised the study?
· Was the study is organised by a company or people with a vested interest in getting a certain result.
e.g. CSR organising a study on whether too much sugar is bad for your health.


Who conducted the study?
·…

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It's All About Variables! 5


VARIABLES ­ Things that vary.
Categorical ­ can contain any value from a range of values. e.g. Sex = F, M (or 0 for F and 1 for M), range of
10,20,30,40 = each time it can contain any one of these numbers.
Metric ­…

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It's All About Variables! 6


Matched Pairs Design

use pairs of participants carefully matched on all major subject nuisance variables
for each pair, one is randomly assigned to each of the treatment conditions
use this avoids practice effects, but
you need to identify all major nuisance variables and find several…

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It's All About Variables! 7


What's Normal?


Example: 70 out of 1000 said yes.


Proportion = Frequency/ Total Number
70 / 1000 = 0.07

Percentage = proportion x 100
0.07 x 100 = 7%



The normal distribution is described as a "symmetric, bell shaped curve". For any given mean and…

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It's All About Variables! 8


Significance Theory

P value - Significance
Tells us what the chance is of getting a proportion as high/low in the sample if the proportion was still the
known proportion.

0.1 = 1 chance in 10, 0.01 = 1 chance in 100, 0.001 = 1 chance…

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It's All About Variables! 9




EXERCISE 3.2 from Foundations of Statistics, 2nd Edition
In 2006, 62% of all AFL players lived in Victoria. Suppose we took a random sample of 25 AFL players from this
population.

We would expect the sample
proportion of AFL players who
lived in Victoria to…

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It's All About Variables! 10



Tests for Categorical Variables
Describing the distribution of a single categorical variable.
SPSS ­ Analyze/Descriptive Statistics/Frequencies
Use Pie chart for 2 or 3 categories
Use Bar chart for 3 or more categories




What you need to know.

Total of participants in sample = 416. Always…

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