Chapter 6 Sampling
Sample: the process of selecting a sample of individuals that represent a larger group.
Population – larger group to which the results can be applied and is the group of interest to the
researcher.
Target Population- is the actual population to whom the researcher would like to generalize
PG 91 offer two examples (Right column)
Random Sampling – (Selecting names from a hat) example - 25 names are picked out of a possible 150
To select a representative sample of the population
Nonrandom – purposive sampling, each member does not have an equal chance of being selected, they
must possess certain criteria to be selected.
Random Sampling Methods
 Simple Random Sample – each and every member of the population has an equal chance of
being selected.
 Stratified Random Sampling – is a process in which certain subgroups or strata are selected for
the sample in the same proportion as they exist in the population
 Cluster Random Sample – the selection of groups or clusters of subjects rather than individuals.
More effective with larger numbers of clusters. Groups rather than individuals are selected
 Two Stage Random Sampling – combination of random and cluster sampling.
Nonrandom Sampling
 Systematic – every nth individual is selected for inclusion in the sample. Example: Poluation list
of 5000 names, sample of 500. The researcher would select every tenth name. There is a
random start, sampling interval, sampling ratio, and periodicity.
 Convenience Sampling – a group of individuals who (conveniently) are available for a study.
Examples: A high school counselor interviews all the students who come to hi m for counseling
about their career plans.
 Purposive Sample – based on previous knowledge of a population, investigator uses personal
judgement to select a sample that they believe will provide the data they need. (Pg 99)
examples
Sampling Methods – “Students with low self-esteem demonstrate lower achievement in school
subjects.”
Target pop: All 8th graders in California
Accessible pop: All 8th graders in SF Bay area
Feasible pop: n =200 -250
Simple Random Sampling – assign each student a number and then use a table of random numbers to
select 200
Cluster Random Sample – Assign each school a number and select 4 schools and include all 8th grade
classes in each school
Stratified Random Sample – randomly select samples of 160 and 40 subpopulations of public and private
students (Pg 100)
Two Stage RS – Randomly select 25 schools then randomly select 8, 8th grade students from each school
Convenience Sampling – select schools which the researcher has access\
Purposive Sampling – use demographic data that’s representative of 8th graders
Systematic Sampling – Select every 45th student from an alphabetic list from each school
Sample Size: Should be as large as the researcher can obtain with a reasonable expenditure of time and
energy. A recommended minimum number of subjects is 100 for a descriptive study, 50 for a
correlation study and 30 in each group for experimental and casual comparative studies.
External Validity – refers to the extent to which the results of a study can be generalized from a sample
to a population.
Population Generalizability – refers to the degree to which a sample represents the population of
interest
Survey Research Chapter 17:
Surveys: basic characterisitics: the collection of information, from a sample, by asking questions, in
order to describe some aspect of the population of which the sample is a part.
Why are surveys conducted? To describe the characteristics of a population (for example, age, religious
preference, attitude toward school)
Cross sectional – collects information from a sample that has been drawn from a predetermined
population. Info is collected at just one point in time.
Longitudinal Survey – info collected at different points in time. Thre types are trend studies, cohort
studies, and panel studies.
Trend – different samples from a population whose members change are surveyed at different points of
time
Cohort- different samples from a population who’s members do not change are surveyed at different
points of time.
Panel – the same samples of individuals is surveyed at different times over the course of the survey
Steps In Survey Research –
Define the problem – researchers need to define clearly the objectives in conducting the survey
Identify the Target Population – the group of persons that is the focus of the study
Modes of Data Collection:
Direct administration
Mail Survey
Telephone Survey
‘
Personal Interviews
The sample to be surveyed must be selected randomly
Most Common: QUestionaire and the interview schedule
Types of Questions used in Surveys:
Open-ended questions
Closed-ended questions
Contingency Questions – a question whose answer is contigent upon how a respondent answers
a prior question.
Interviewing – obvioiusly telephone and face-to-face interviewers need to be trained prior to
administering
Major Problem – total nonresponse and item nonresponse.
Main Threats to Internal Validity
1. Mortality
2. Location
3. Instrumentation
4. Instrument decay
An example of Survey research – Pgs 405-413