Discussion Write a 175- to 265-word response to the following questions:
Why should multiple sampling methodologies be considered for health care research? Explain.
What are the risks of sampling errors? 1114237 – Wiley US ©
INTRODUCTION INTRODUCTION 161161
THE THE NANATURE TURE OF OF SAMPLES SAMPLES 163163
OVEROVERVIEWVIEW OF OF PROBABILITY PROBABILITY SAMPLING SAMPLING 164164
PROBABILITYPROBABILITY SAMPLING SAMPLING METHODS METHODS 167167
NONPROBABILITYNONPROBABILITY SAMPLING SAMPLING 170170
CENTRALCENTRAL LIMIT LIMIT THEOREM THEOREM 173173
APPLICAAPPLICATIONS TIONS OF OF THE THE CENTRAL CENTRAL LIMIT LIMIT THEOREM THEOREM 177177
SOURCES SOURCES OF OF BIAS BIAS AND AND ERROR: ERROR: AA REPRISE REPRISE 183183
RESEARCH RESEARCH IN IN ACTION: ACTION: SAMPLING SAMPLING MAMATTERS TTERS 186186
It It is is a a capital capital mistake mistake to to theorize theorize before before one one has has data.data.
—Sir —Sir Arthur Arthur Conan Conan DoyleDoyle
Much Much of of what what we we know know we we learn learn from from studying studying samples samples of of larger larger “populations.” “populations.”
Consider Consider some some common common examples. examples. When When someone someone asks asks you you whether whether the the lunches lunches
at at a a restaurant restaurant are are good, good, you you use use your your experiences experiences to to answer answer the the question. question. The The
meals meals you you have have eaten eaten at at that that restaurant restaurant represent represent a a sample sample of of all all the the possible possible meals meals
that that you you and and others others could could have have eaten eaten at at that that restaurant. restaurant. The The establishment establishment may may
produce produce thousands thousands of of meals meals and and has has served served hundreds hundreds of of customers customers since since its its
opening. opening. YYou ou have have no no way way of of being being sure sure that that each each meal meal was was prepared prepared well well and and
enjoyed. enjoyed. YYet, et, based based on on your your experience experience with with the the restaurant, restaurant, you you make make a a single single and and
confident confident conclusion, conclusion, “The “The food food here here is is good.”good.”
Understanding Understanding and and Conducting Conducting Research Research in in the the Health Health SciencesSciences, , First First Edition. Edition. Christopher Christopher J. J. L. L. Cunningham, Cunningham,
Bart Bart L. L. WWeathington, eathington, and and David David J. J. PittengerPittenger..
© © 2013 2013 John John Wiley Wiley & & Sons, Sons, Inc. Inc. Published Published 2013 2013 by by John John Wiley Wiley & & Sons, Sons, Inc.Inc.
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162162 Chapter Chapter 7 7 SamplingSampling
The The same same is is true true when when you you meet meet a a person person for for the the first first time. time. Social Social psycholo-psycholo-
gists gists telltell us us that that we we make make long-lasting long-lasting conclusions conclusions based based on on our our first first impressions impressions
of of people. people. Think Think for for a a moment moment of of what what happens happens in in such such situations. situations. After After a a mere mere
five five minutes, minutes, many many people people are are willing willing to to state state confidently confidently that that they they have have formed formed
a a good good impression impression of of another another person; person; that that is, is, an an extremely extremely small small sample sample of of behav-behav-
ior ior creates creates a a lasting lasting impression impression of of that that person. person. Given Given the the complexity complexity of of human human
behaviorbehavior, , this this is is an an amazing amazing conclusion conclusion based based on on such such a a limited limited sample.sample.
As As you you will will recall recall from from Chapter Chapter 1, 1, Bacon Bacon warned warned about about the the idols idols of of the the cave, cave,
relying relying too too much much on on our our personal personal experience experience for for seeking seeking truth. truth. Objective Objective or or sci-sci-
entific entific sampling sampling is is a a way way of of overcoming overcoming the the idols idols of of the the cave cave because because it it ensures ensures
that that our our experiences experiences are are as as broad broad and and representative representative as as possible. possible. MoreoverMoreover, , as as you you
learned learned in in Chapter Chapter 3, 3, having having a a representative representative sample sample of of the the population population in in a a study study
helps helps to to ensure ensure the the external external validity validity of of the the conclusions conclusions we we draw draw from from the the data data to to
other other members members of of the the population.population.
What What are are the the common common features features of of samples samples and and sampling? sampling? First, First, we we use use samples samples
to to describe describe and and compare compare things. things. ConsiderConsider, , as as an an example, example, the the challenge challenge of of describ-describ-
ing ing someone’someone’s s personalitypersonality. . Each Each of of us us is is affected affected by by many many things, things, ranging ranging from from
situational situational factors factors to to interactions interactions with with others, others, and and myriad myriad other other variables variables too too
numerous numerous to to list. list. Even Even for for an an individual, individual, it it would would be be impossible impossible to to observe observe all all the the
things things he he or or she she would would do do under under different different situations. situations. However, However, based based on on a a repre-repre-
sentative sentative sample sample of of that that person’person’s s behaviorbehavior, , we we may may be be able able to to predict predict how how he he or or
she she will will react react to to specific specific situations. situations. This This observation observation leads leads us us to to the the second second feature feature
of of sampling.sampling.
WWe e use use samples samples to to help help us us make make predictions predictions and and conclusions conclusions about about other other
things things or or conditions. conditions. As As the the quotation quotation at at the the start start of of this this chapter chapter suggests, suggests, our our
theories theories are are the the product product of of the the data data we we collect. collect. Perhaps, Perhaps, by by extension, extension, we we can can also also
suggest suggest that that no no theory theory is is any any better better than than the the data data that that either either inspired inspired its its creation creation
or or are are used used to to test test it.it.
How How can can we we be be sure sure tthat hat what what we we observe observe in in a a sample sample accurately accurately represents represents
the the broader broader population? population? This This is is a a critical critical question. question. As As you you learned learned in in Chapter Chapter 3, 3,
such such a a question question is is at at the the heart heart of of external external validityvalidity. . If If the the sample sample is is not not representa-representa-
tive tive of of the the population, population, then then the the data data will will be be of of little little value value to to us us because because they they do do
not not support support inferences inferences about about the the population.population.
Therefore, Therefore, the the purpose purpose of of this this chapter chapter is is to to examine examine the the foundations foundations of of samples samples
and and sampling. sampling. In In the the following following pages, pages, we we will will examine examine the the methods methods researchers researchers
use use to to collect collect accurate accurate samples samples of of the the population. population. In In addition, addition, we we will will review review how how
statistics, statistics, based based on on sample sample data, data, allow allow us us to to make make inferences inferences about about population population
parameters. parameters. Hence, Hence, we we will will begin begin with with a a more more detailed detailed analysis analysis of of the the charactercharacter–
istics istics of of samples. samples. WWe e assume assume that that you you have have had had a a course course in in statistics statistics and and under-under-
stand stand the the foundations foundations of of descriptive descriptive statistics statistics such such as as measures measures of of central central tendency tendency
(e.g., (e.g., mean mean and and median), median), measures measures of of dispersion dispersion (e.g., (e.g., variance variance and and standard standard
deviation deviation [[ ]), ]), and and basic basic standard standard or or -scores. -scores. If If you you are are not not familiar familiar with with these these SDSD zz
statistics, statistics, or or wish wish to to brush brush up up on on your your skills, skills, you you might might benefit benefit from from reviewing reviewing
Appendix Appendix A.A.
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THE THE NATURE NATURE OF OF SAMPLESSAMPLES
There There are are several several ways ways that that we we can can define define samples. samples. The The primary primary feature feature of of a a
“good” “good” sample sample for for research research is is that that the the sample sample actually actually represents represents that that population population
from from which which it it was was selected. selected. One One way way to to define define a a sample sample is is to to contrast contrast samples samples
with with things things that that are are not not samples. samples. WWe e can can also also define define samples samples by by the the methods methods used used
to to create create them. them. As As you you may may recall recall from from our our previous previous discussions discussions of of validityvalidity, , we we
cannot cannot directly directly assess assess the the external external validity validity or or the the representativeness representativeness of of a a sample. sample.
RatherRather, , we we examine examine the the methods methods used used to to create create the the sample sample to to infer infer its its validityvalidity. .
ConsequentlyConsequently, , researchers researchers work work hard hard to to ensure ensure that that their their sampling sampling techniques techniques
produce produce useful useful data.data.
Scientific Scientific Samples Samples versus versus Anecdotal Anecdotal EvidenceEvidence
What What is is the the difdifference ference between between sample-based sample-based data data and and other other types types of of infoinformation? rmation?
WWe e can can begin begin by by considering considering personal personal experiences experiences and and anecdotal anecdotal evidence evidence and and
then then compare compare them them to to a a scientific scientific sample. sample. Imagine Imagine a a person person who who traveled traveled to to Paris Paris
for for a a short short vacation vacation and and then then pronounces pronounces the the French French to to be be rude rude and and inconsiderate inconsiderate
people. people. Such Such a a conclusion conclusion is is indefensible. indefensible. AA moment’moment’s s thought thought will will reveal reveal that that
many many factors factors bias bias this this unflattering unflattering conclusion. conclusion. SpecificallySpecifically, , a a stranger stranger traveling traveling
abroad, abroad, spending spending a a short short time time in in a a small small portion portion of of the the countrycountry, , and and interacting interacting
with with a a minuscule minuscule proportion proportion of of the the population, population, cannot cannot come come to to a a meaningful meaningful con-con-
clusion clusion about about the the manners manners of of a a nation nation of of people.people.
Anecdotal Anecdotal evidenceevidence and and personal personal experience experience are are not not sampling sampling in in the the technical technical
sense sense of of the the word. word. Our Our hypothetical hypothetical traveler traveler probably probably did did meet meet several several rude rude and and
inconsiderate inconsiderate people, people, an an experience experience likely likely to to occur occur when when traveling traveling to to any any large large
citycity. . Therefore, Therefore, we we cannot cannot deny deny the the person’person’s s experiences. experiences. WWe e do do deny deny the the validity validity
of of the the conclusion, conclusion, howeverhowever, , because because we we do do not not consider consider that that one one person’person’s s encoun-encoun-
ters ters to to be be representative representative of of the the population.population.
WWe e can can use use the the criteria criteria of of representativeness representativeness to to distinguish distinguish anecdotal anecdotal evidence evidence
and and personal personal experience experience from from scientific scientific samples samples and and empirical empirical conclusions. conclusions. The The
goal goal of of scientific scientific research research is is to to identify identify and and collect collect data data from from samples samples of of partici-partici-
pants pants that that represent represent the the population population that that we we studystudy. . In In an an experimental experimental scenario, scenario,
researchers researchers are are also also interested interested in in creating creating treatment treatment groups groups that that are are equivalent equivalent on on
all all variables variables other other than than the the independent independent variable(s) variable(s) of of interest. interest. From From these these samples samples
and and treatment treatment groups, groups, researchers researchers collect collect data data and and then then have have to to generate generate reasonable reasonable
and and valid valid conclusions conclusions regarding regarding these these data. data. When When we we collect collect the the data, data, we we use use
procedures procedures that that ensure ensure that that the the samples samples will will be be free free of of bias bias and and thereby thereby represent represent
the the population. population. By By contrast, contrast, anecdotal anecdotal evidence evidence is is typically typically a a haphazard haphazard collection collection
of of personal personal experiences experiences that that may may be be biased.biased.
UnfortunatelyUnfortunately, , anecdotal anecdotal evidence evidence too too easily easily sways sways many many pepeople’ople’s s opinions, opinions, an an
observation observation that that Bacon Bacon noted noted when when he he described described the the idols idols of of the the tribe tribe and and idols idols of of
the the cave. cave. Social Social psychologists psychologists have have long long studied studied the the poor poor decision-making decision-making
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processes processes that that many many people people follow follow (Plous, (Plous, 1993). 1993). AA well-known well-known phenomenon phenomenon is is the the
“person-who” “person-who” effect effect that that occurs occurs when when someone someone uses uses anecdotal anecdotal evidence evidence to to discount discount
a a statistical statistical generalization. generalization. For For example, example, a a smoker smoker may may dismiss dismiss the the risk risk of of smoking smoking
by by noting noting that that his his or or her her father father smoked smoked two two packs packs of of cigarettes cigarettes a a day day and and lived lived to to
be be 75. 75. In In this this case, case, the the person person seems seems to to ignore ignore the the larger larger body body of of evidence evidence that that
people people who who smoke smoke have, have, on on average, average, a a shorter shorter life life expectancy expectancy and and an an increased increased risk risk
of of health health problems.problems.
The The problem problem we we all all must must confront confront in in research research and and in in life life is is the the pervasive pervasive nature nature
of of anecdotal anecdotal evidence evidence and and its its influence influence on on decision decision making. making. As As researchers, researchers, we we
must must remain remain vigilant vigilant to to ensure ensure that that our our data data are are representative representative of of the the population. population.
FinallyFinally, , you you should should recognize recognize that that the the plural plural of of is is not not . . Anecdotes, Anecdotes, anecdoteanecdote datadata
even even a a whole whole collection collection of of them, them, like like rumors, rumors, cannot cannot be be trusted trusted to to be be accurate, accurate,
reliable, reliable, or or valid.valid.
OVERVIEW OVERVIEW OF OF PROBABILITY PROBABILITY SAMPLINGSAMPLING
Before Before we we delve delve into into the the topic topic of of sampling sampling any any furtherfurther, , we we must must define define an an impor-impor-
tant tant term. term. TTo o say say that that an an outcome outcome is is means means that that each each possible possible outcome outcome randomrandom
has has an an equal equal probability probability of of occurring occurring and and that that the the outcome outcome of of one one event event has has no no
influence influence on on the the probability probability of of subsequent subsequent outcomes. outcomes. Imagine Imagine a a balanced balanced six-six-
sided sided die. die. Each Each time time you you throw throw the the die, die, the the probability probability of of any any one one side side landing landing
faceup faceup is is always always one-sixth. one-sixth. In In addition, addition, each each time time you you throw throw the the die, die, the the results results
of of the the previous previous tosses tosses have have no no effect effect on on the the current current toss. toss. That That one one of of these these events events
has has no no effect effect on on the the other other possible possible events events is is an an illustration illustration of of the the principle principle of of
independenceindependence. . Random Random sampling sampling is is also also an an example example of of independence independence because because
when when used used in in sample sample selection, selection, selecting selecting one one object object from from the the population population will will not not
influence influence the the selection selection of of other other objects.objects.
Understanding Understanding randomness randomness is is important important for for sampling sampling because because it it allows allows us us to to
link link individual individual samples samples to to the the population. population. According According to to the the law law of of large large numbersnumbers, ,
any any larlarge ge number number of of items items chosen chosen at at random random from from a a population population will will have, have, on on
average, average, the the same same characteristics characteristics as as the the population. population. This This law law is is the the cornerstone cornerstone of of
probability probability sampling sampling and and allows allows us us to to infer infer that that what what is is true true of of a a randomly randomly selected selected
sample sample is is also also true true of of the the population. population. In In this this section, section, we we will will examine examine how how we we can can
use use random random events events and and other other procedures procedures to to create create samples samples that that represent represent the the
Populations Populations and and SamplesSamples
In In Chapter Chapter 3, 3, we we introduced introduced you you to to the the concepts concepts of of the the population population and and the the sample. sample.
As As you you should should recall, recall, the the population population consists consists of of all all the the individuals individuals or or things things that that
the the researcher researcher wants wants to to describe. describe. Researchers Researchers define define the the population population by by developing developing
the the criteria criteria that that determine determine membership membership in in the the population. population. Defining Defining the the population population
is is essential essential as as it it determines determines the the conclusions conclusions that that the the researcher researcher may may draw draw from from the the
data data (W(Wilkinson, ilkinson, 1999).1999).
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When When referring referring to to a a population, population, many many researchers researchers refer refer to to the the target target popula-popula-
tiontion universeuniverse or or the the . . For For example, example, a a medical medical researcher researcher may may want want to to study study the the
rates rates of of obesity obesity among among teenagers teenagers between between the the ages ages of of 13 13 and and 18. 18. In In this this example, example,
the the target target population population is is children children between between the the ages ages of of 13 13 and and 18, 18, inclusive.inclusive.
In In some some cases, cases, it it is is impractical impractical or or impossible impossible to to draw draw a a representative representative sample sample
from from the the tartarget get population. population. ConsequentlyConsequently, , researchers researchers will will draw draw their their sample sample from from
a a smaller smaller subset subset of of the the population population called called a a . . For For our our pur-pur-sampling sampling populationpopulation
poses, poses, a a sampling sampling population population consists consists of of an an accessible accessible group group of of people people who who share share
the the same same characteristics characteristics as as the the target target population. population. In In most most health health sciences sciences research, research,
the the sampling sampling population population consists consists of of people people who who live live or or work work near near the the researcherresearcher. .
When When they they report report their their results, results, researchers researchers define define the the characteristics characteristics of of the the sam-sam-
pling pling population. population. Doing Doing so so allows allows the the reader reader to to determine determine wwhether hether the the sampling sampling
population population is is representative representative of of the the target target population.population.
Some Some researchers researchers pull pull their their study study samples samples from from an an existing existing , , a a subject subject poolpool
group group of of people people to to whom whom the the researcher researcher has has easy easy access. access. At At many many colleges colleges and and
universities, universities, the the academic academic departments departments maintain maintain a a subject subject pool pool of of students students enrolled enrolled
in in introductory-level introductory-level courses courses who who may may receive receive extra extra credit credit for for participating participating in in a a
studystudy. . In In other other cases, cases, people people who who have have a a known known medical medical condition, condition, for for example, example,
may may agree agree to to have have their their name name added added to to a a list list of of potential potential participants participants for for controlled controlled
AA related related concept concept is is the the sampling sampling frameframe, , which which is is the the set set of of individuals individuals within within
a a population population who who can can actually actually be be reached reached for for a a specific specific research research purpose, purpose, for for
example, example, a a list list of of names names that that identifies identifies the the members members of of the the population population is is a a sampling sampling
frame frame (Babbie, (Babbie, 1998). 1998). Consider Consider a a study study conducted conducted by by WWashington, ashington, Sun, Sun, and and
Canning Canning (2010) (2010) in in which which they they created created a a sampling sampling frame frame to to better better examine examine medical medical
care care received received by by veterans. veterans. As As they they noted noted in in their their studystudy, , most most veterans veterans do do not not
receive receive health health care care from from the the Department Department of of VVeterans eterans Affairs Affairs (V(VA). A). Therefore, Therefore, using using
data data drawn drawn exclusively exclusively from from VVAA data data sets sets may may be be incomplete incomplete and and biased. biased. TTo o
address address this this potential potential problem, problem, they they created created a a sampling sampling frame frame using using data data from from the the
VVeterans eterans Health Health Administration, Administration, VVeterans eterans Benefits Benefits Administration, Administration, and and Depart-Depart-
ment ment of of Defense. Defense. WWashington ashington et et al. al. found found that that the the sampling sampling frame frame provided provided a a
wider wider range range of of cases cases than than would would have have been been examined examined had had they they used used only only the the VVAA
data data set. set. Therefore, Therefore, samples samples generated generated using using this this sampling sampling frame frame provides provides
researcher researcher access access to to data data that that are are more more likely likely to to generalize generalize to to the the health health care care and and
status status of of male male and and female female veterans.veterans.
Detailed Detailed descriptions descriptions of of the the characteristics characteristics of of the the population, population, sampling sampling popu-popu-
lation, lation, and and sampling sampling frame frame help help us us evaluate evaluate the the external external validity validity of of inferences inferences
made made from from a a sample. sample. As As you you have have seen seen in in the the previous previous example, example, WWashington ashington et et
al. al. (2010) (2010) were were concerned concerned that that researresearch ch based based exclusively exclusively on on VVAA data data would would be be
incomplete incomplete and and not not representative representative of of the the population population of of veterans. veterans. By By expanding expanding the the
sampling sampling frame frame to to include include other other data data sets, sets, they they demonstrated demonstrated that that samples samples from from
this this sampling sampling frame frame would would be be far far more more inclusive inclusive and and representative representative of of the the popula-popula-
tion tion of of American American veterans.veterans.
The The connection connection among among external external validity validity and and the the sampling sampling population, population, subject subject
pool, pool, and and sampling sampling frame frame is is important. important. If If any any of of these these subgroups subgroups is is not not
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representative representative of of the the population, population, then then this this threatens threatens the the external external validity validity of of any any
conclusions conclusions that that are are based based on on the the data data collected collected from from the the sample. sample. For For these these
reasons, reasons, researchers researchers spend spend considerable considerable time time designing designing careful careful sampling sampling plans plans and and
analyzing analyzing and and describing describing the the sources sources of of their their data. data. Doing Doing so so allows allows the the researcher researcher