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
INTRODUCTIONINTRODUCTION
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 re
staurant 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.”
161161
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.