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Population is the total group of interest
target population is the group to whom researchers hope
to generalize their findings.
– accessible population is the group of potential research
subjects who are actually available for a given study.
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Sampling is the process by which a subgroup of
subjects is selected for study from a larger group of
potential subjects (population).
Sampling methods influence the characteristics of
the sample, which in turn influence the
generalizability, or external validity, of a piece of
research.
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Once the researcher has defined the accessible
population in a general way,
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you needs to develop more specific inclusion and
exclusion characteristics.
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Simple Random Sampling
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Drawing a sample from a cage, or even from a hat,
may work fairly well when the accessible population
is small.
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The preferred method for generating a simple
random sample is to use random numbers that are
provided in a table or generated by a computer.
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Systematic sampling is a process by which the
researcher selects every (…th) person on a list.
For example:
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We choose every ninth person or tenth person of a
certain population.
Or using medical record number, license number,
date of birth,……
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In a stratified sample the sampling frame is
divided into non-overlapping groups or strata,
e.g. geographical areas, age-groups, genders.
A sample is taken from each stratum, and
when this sample is a simple random sample it
is referred to as stratified random sampling.
For example:
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if you were interested in Internet usage you might
stratify by age
if you were interested in smoking you might stratify
by gender or social class.
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Stratified sampling is used when certain
subgroups must be represented in adequate
numbers within the sample or when it is
important to preserve the proportions of
subgroups in the population within the sample.
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For example:
If I want to study the total knee replacement in 8 hospitals
Each hospital has different numbers of TKR per year
So I need to have an equal and fair representation of each hospital in
my sample
My sample will be 10% of the population, and the same percent from
each hospital.
This some time called proportional allocation.
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Cluster sampling is the use of naturally
occurring groups as the sampling units.
For example, if we wanted to conduct a
nationwide study.
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first sample states, then cities within each selected
state, then hospitals within each selected city, and
then patients within each selected hospital.
Each step could be implemented through simple
random, systematic, or stratified sampling.
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Sampling without randomization.
The reason is limited funding.
subject selection is confined to a available
patients
Three forms of nonprobability sampling are:
1.
2.
3.
convenience
Snowball
purposive sampling.
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Samples of convenience involve the use of readily available
subjects in certain diagnostic categories at a single clinic.
nonprobability method is often used during preliminary research
efforts to get a gross estimate of the results
Consecutive sampling is a form of convenience sampling.
Consecutive samples are used in a prospective study in which the
population does not exist at the beginning of the study
–
all patients who meet the criteria are placed into the study. This
continues until a specified number of patients is collected, a
specified time frame has passed, or certain statistical outcomes are
seen.
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A snowball sample may be used when the
potential members of the sample are difficult to
identify or rare.
In a snowball sample researchers identify a few
subjects who are then asked to identify other
potential members of the sample.
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Purposive sampling is used when a researcher
has a specific reason for selecting particular
subjects for study.
A researcher wants to get opinions from nonworking mothers. They go around an area
knocking on doors during the day. Their first
questions are then about whether there are
children and whether the woman has a day job.
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Assignment is the process by which subjects in
the sample are assigned to groups within the
study.
Random assignment to groups is preferred and
is appropriate even when the original selection
procedure was nonrandom.
assignment process is often to develop groups
of equal size.
Research MU 1433 Dr. Salameh Al
Dajah
1.
Random Assignment by Individual
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2.
This could be done with a roll of a die.
Disadvantage not equal group sizes.
Random Assignment by Block
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to ensure equal group sizes
For example, assigning four groups of eight subjects
(using random numbers table).
3.
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Systematic Assignment
Researchers count off by using a list of the
sample and systematically placing subsequent
subjects into subsequent groups.
For example, The first person was assigned to
the 1st group, the second person to 2nd group,
the third person to 3rd group, the fourth person
to 4th group, the fifth person to the 1st , and so
on.
4.
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5.
Matched Assignment
In matched assignment, subjects are matched
on important characteristics and these
subgroups are randomly assigned to study
groups.
Consecutive Assignment