chapter 9
DESCRIPTION
Chapter 9. Examining Populations and Samples in Research. Sampling Concepts. Sampling: Selecting a group of people, events, behaviors, or other elements with which to conduct a study Sampling plan: Sampling method; defines the selection process - PowerPoint PPT PresentationTRANSCRIPT
1Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Chapter 9
Examining Populations and Samples in Research
2Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Concepts
Sampling: Selecting a group of people, events, behaviors, or other elements with which to conduct a study
Sampling plan: Sampling method; defines the selection process
Sample: Defines the selected group of people or elements from which data are collected for a study
Members of the sample can be called the subjects or participants.
3Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Populations and Elements
Population: A particular group of individuals or elements who are the focus of the research
Target population: An entire set of individuals or elements who meet the sampling criteria
Accessible population: The portion of the target population to which the researcher has reasonable access
Elements: Individual units of the population and sample
4Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Generalization
Extending the findings from the sample under study to the larger population
The extent is influenced by the quality of the study and consistency of the study’s findings.
5Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Criteria: Inclusion
Characteristics that the subject or element must possess to be part of the target population
Examples: Between the ages of 18 and 45 Ability to speak English Admitted for gallbladder surgery Diagnosed with diabetes within past month
6Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Criteria: Exclusion
Characteristics that can cause a person or element to be excluded from the target population
Examples: Diagnosis of mental illness Less than 18 years of age Diagnosis of cognitive dysfunction Unable to read or speak English
7Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Defining Sampling Criteria
Homogeneous sample: As similar as possible so as to control for extraneous variables
Heterogeneous sample: Represents a broad range of values Used when a narrow focus is not desirable
8Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Inappropriate Generalizations
Samples cannot be generalized beyond their sampling criteria.
This may lead to inappropriate generalizations: Because of language or reading ability To other types of illnesses or injuries
9Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Representativeness
The sample, the accessible population, and the target population are alike in as many ways as possible.
Need to evaluate: Setting Characteristics of subjects (age, gender, ethnicity,
income, education) Distribution of values on variables measured in the
study
10Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Error
Difference between the population mean and the mean of the sample
Random variation The expected difference in values that occurs
when different subjects from the same sample are examined
Difference is random because some values will be higher and others lower than the average population values
11Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling error
Population Sample
Populationmean
Samplemean
Sampling Error (cont’d)
12Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Error (cont’d)
Systematic variation (bias) Consequence of selecting subjects whose
measurement values differ in some specific way from those of the population
These values do not vary randomly around the population mean.
13Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Random vs. Systematic Variation in Sampling
Random variation: Expected difference in values that occurs when different subjects from same sample are examined Difference is random because some values will be
higher or lower than the mean population value. As sample size increases, random variation
decreases. Systematic variation (or systematic bias):
Consequence of selecting subjects whose measurement values differ in some way from those of the population
14Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Refusal Rate vs. Acceptance Rate
Refusal rate: Percentage of subjects who declined to participate in the study 80 subjects approached and 4 refused 4 80 = 0.05 = 5% refusal rate
Acceptance rate: Percentage of subjects who consented to be in the study 80 subjects approached and 76 accepted 76 80 = 0.95 = 95% acceptance rate
15Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sample Attrition and Retention
Sample attrition: Withdrawal or loss of subjects from a study Attrition rate = number of subjects withdrawing ÷
number of study subjects × 100 Sample retention: Number of subjects who
remain in and complete a study.
16Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Random Sampling
Increases the representativeness of the sample based on the target population
Control group: Used in studies with random sampling
Comparison group: Not randomly determined
17Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sampling Frame and Sampling Plan
Sampling frame: A listing of every member of the population, using the sampling criteria to define membership in the population
Subjects are selected from the sampling frame
Sampling plan: Outlines strategies used to obtain a sample for a study Probability sampling plans Nonprobability sampling plans
18Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Types of Probability Sampling
Simple random sampling Stratified random sampling Cluster sampling Systematic sampling
19Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Simple Random Sampling
Randomly choosing the sample Can use a table of random numbers Can draw names out of a hat
20Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Stratified Random Sampling
Ensures all levels of identified variables are adequately represented in the sample
Needs a large population with which to start Variables often stratified
Age, gender, socioeconomic status Types of nurses, sites of care
21Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Cluster Sampling
All areas with the elements of the identified population are linked.
A randomized sample of these areas is then chosen.
Used to get a geographically diverse sample Also used when developing a sampling frame
is difficult because of a lack of knowledge of the variables
22Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Systematic Sampling
Selecting every kth individual on the list, starting randomly
Researcher must know number of elements in the population and the sample size desired
23Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Nonprobability Sampling
Quantitative research Convenience (accidental) sampling Quota sampling
24Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Convenience Sampling
Also called accidental sampling Weak approach to sampling because it is
hard to control for bias The sample includes whomever is available
and willing to give consent. Representativeness is a concern.
25Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Quota Sampling
Uses convenience sampling, but with a strategy to ensure inclusion of subject types who are likely to be underrepresented in the convenience sample
Goal is to replicate the proportions of subgroups present in the population
Works better than convenience sampling to reduce bias
26Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sample Size in Quantitative Studies
Affect size Type of quantitative study conducted Number of variables Measurement sensitivity Data analysis techniques
27Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Power Analysis
Ability to detect differences in the population or capacity to correctly reject a null hypothesis
Standard power of 0.8 Level of significance
Alpha = 0.05, 0.01, 0.001
28Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Effect Size
The effect is the presence of the phenomenon being studied.
The effect size is the extent to which the null hypothesis is false.
When the effect size is large (large variation between groups), only a small sample is needed.
Increasing the sample size increases the effect size.
29Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Number of Variables
As the number of variables increases, the sample size may increase.
The inclusion of multiple dependent variables also increases the sample size needed.
30Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Measurement Sensitivity
Was the tool used a reliable and valid measure of the variable?
As the variance in the instrument scores increases, the sample size needed to obtain significance increases.
31Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Data Analysis Techniques
ANOVA and t-test require equal group sizes, which will increase power because the effect size is maximized.
Chi-square is the weakest of the tests and requires a large sample size to achieve acceptable levels of power. As the number of categories increases, the
sample size must increase as well.
32Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Critiquing the Sample
Identify Elements Accessible population Target population
Evaluate Appropriateness of generalization in quantitative
studies
33Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Critiquing the Sample (cont’d)
Identify the sample criteria. Judge appropriateness of the sampling
criteria. Identify the sampling method.
34Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Nonprobability Sampling
Qualitative research Purposive sampling Network or snowball sampling Theoretical sampling
35Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Purposeful or Purposive Sampling
Also called judgmental or selective sampling Efforts are made to include typical or atypical
subjects. Sampling is based on the researcher’s
judgment.
36Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Network Sampling
Also called snowball sampling Takes advantage of social networks to get
the sample One person in the sample asks another to
join the sample, and so on.
37Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Theoretical Sampling
Used in grounded theory research Data are gathered from any individual or
group that can provide relevant data for theory generation.
The sample is saturated when the data collection is complete based on the researchers’ expectations.
Diversity in the sample is encouraged.
38Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Sample Size in Qualitative Research
Scope of the study Nature of the topic Quality of the data Study design
39Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Scope of the Study
Broad studies require larger samples than narrow studies.
The sample size must be adequate for the scope.
40Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Nature of the Topic
If the study topic is clear, fewer subjects are needed.
If the topic is difficult to define, then a larger sample is needed.
41Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Quality of the Data
How rich are the data? Were data collected from the best sources?
42Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Study Design
How many interviews were carried out? Was the design adequate for the variables?
43Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Adequacy of the Sample in Qualitative Studies
Are the sampling inclusion and exclusion criteria appropriate?
Is the sampling plan adequate to address the purpose of the study?
Is the sample size adequate? What are the refusal and mortality rates?
44Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Adequacy of the Sample in Qualitative Studies (cont’d)
Are sample characteristics and quality described?
Is there saturation of the data? Is the setting defined?
45Copyright © 2011 by Saunders, an imprint of Elsevier Inc.
Research Settings
Natural or field setting: uncontrolled in real life Seen in descriptive or correlational studies
Partially controlled setting: manipulated or modified by the researcher Seen in correlational, quasi-, or experimental
studies Highly controlled setting: artificially
constructed by researcher (i.e., lab setting) Seen in experimental studies