Copyright c 2001 The McGraw-Hill Companies, Inc.1 Chapter 7 Sampling, Significance Levels, and...
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Transcript of Copyright c 2001 The McGraw-Hill Companies, Inc.1 Chapter 7 Sampling, Significance Levels, and...
Copyright c 2001 The McGraw-Hill Companies, Inc. 1
Chapter 7
Sampling, Significance Levels, and Hypothesis Testing
Three scientific traditions critical to experimental research Sampling
Significance levels
Hypothesis testing
Copyright c 2001 The McGraw-Hill Companies, Inc. 2
Population and Sample
Population – all units (people or things) possessing the attributes and characteristics of interestSample -- subset of a populationSampling frame -- subset of units that have a chance to become part of the sampleResearchers study the sample to make generalizations back to the population
Copyright c 2001 The McGraw-Hill Companies, Inc. 3
Defining the Population
Choose the dimensions or characteristics meaningful to the hypothesis or research question
Must be at least one common characteristic among all members of a population
Must develop procedure to ensure representative sampling
Copyright c 2001 The McGraw-Hill Companies, Inc. 4
Addressing Generalizability
Extent to which conclusions developed from data collected from sample can be extended to its populationSample is representative to the degree that all units had same chance for being selectedRepresentative sampling eliminates selection bias Characteristics of population should appear to the same
degree in sample
Representativeness can only be assured through random sampling
Copyright c 2001 The McGraw-Hill Companies, Inc. 5
Probability Sampling
The probability of any unit being included in the sample is known and equal
When probability for selection is equal, selection is random
Also known as random sampling
Sampling error will always occur
Copyright c 2001 The McGraw-Hill Companies, Inc. 6
Types of Probability Sampling
Simple random sampling Simplest and quickest
Systematic sampling If used on a randomly ordered frame, results in truly
random sample
Stratified random sampling Random sampling within all subgroups
Cluster sampling Random sampling within known clusters
Copyright c 2001 The McGraw-Hill Companies, Inc. 7
Nonprobability Sampling
Does not rely on random selectionWeakens sample-to-population representativenessUsed when other techniques will not result in an adequate or appropriate sampleUsed when researchers desire participants with special experiences or abilities – including qualitative research
Copyright c 2001 The McGraw-Hill Companies, Inc. 8
Nonprobability Sampling Techniques
Convenience sample
Volunteer sample
Inclusion/exclusion sample
Snowball sample
Network sample
Purposive sample
Quota sample
Copyright c 2001 The McGraw-Hill Companies, Inc. 9
Sample Size
Number of people/units for whom you need to collect data
Determined prior to selecting sample
Less than the number you ask to participate
The larger the sample relative to the population, the less error or bias
Copyright c 2001 The McGraw-Hill Companies, Inc. 10
Comparisons of Sample Size to Population
Population Size
Sample Size
Population Size
Sample Size
100 80 1,000 278
200 132 5,000 357
500 217 50,000 384
Copyright c 2001 The McGraw-Hill Companies, Inc. 11
Significance Levels
The researcher sets the significance level, or p, for each statistical test
The degree of error the researcher finds acceptable in a statistical test
An estimate of what would happen if the study were actually repeated many times
Generally .05 is accepted level
Copyright c 2001 The McGraw-Hill Companies, Inc. 12
Significance Levels
.05 significance level = 5 out of 100 findings that appear to be valid will be due to chance
Also known as the alpha level or p
If p > .05, the finding is nonsignificant
If p is .05, the finding is significant or real
Copyright c 2001 The McGraw-Hill Companies, Inc. 13
Hypothesis Testing
Hypothesis states the expected relationship or difference between two or more variables
Alternative hypothesis presented in report
Null is statistically tested Act of decision making based on the
significance level Decision based on comparison between p set
before study to p produced by statistical test
Copyright c 2001 The McGraw-Hill Companies, Inc. 14
Hypothesis Testing
Belief in the null hypothesis continues until there is sufficient evidence to the contrary
If p for statistical test exceeds significance level, null is retained (p > .05)
If p for statistical test is .05 then alternative hypothesis is accepted
Copyright c 2001 The McGraw-Hill Companies, Inc. 15
Error in Hypothesis Testing
In reality, the null hypothesis is true
In reality, the null hypothesis is false
Use level of significance to reject null
Type I error – Null is rejected even though it is true
Decision 1 – Null is rejected when it is false
Use level of significance to retain the null
Decision 2 – Null is retained when it is true
Type II error – Null is retained even though it is false