Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC CHAPTER 7 Designing the Experiment

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Samples and Populations Population: The entire collection of cases as defined by a set of criteria. - Accessible population: The collection of cases as defined by the criteria and that are available to the investigator. - Target population: The entire collection of cases to which research results (observations or conclusions) are intended to be generalized. Sample: A subset of the population. 2

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Selecting Samples Populations are defined by the researcher and should be clearly specified. Populations need not be large in number. The accessible population from which a sample is drawn should be clearly described by a researcher. Sample size does affect the precision of estimates, given a certain magnitude of treatment effect, and formulas exist for estimating sample size needed to achieve certain risks of error and precision. 3

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Obtaining a Sample Non-probability sampling -nonrandom methods are used to select a sample Probability sampling is based on random sampling- know the probability that each unit in the defined population will be chosen 4

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Random Sampling Simple random sampling- every unit in the population has an equal and independent chance of being selected Stratified sampling - selected by randomly choosing a specified number from each stratum 5

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Random Sampling Systematic sampling-a random sample, sampling interval, K, is simply N/n Cluster sampling – sampling units occur in intact groups or clusters. 6

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Basic Concepts of Research Design Research design is the plan or organization for manipulating, observing, and controlling variables in a research question. Variable: An entity that can take on different values. Independent variable: variable that is manipulated/ the treatment. Dependent variable: variable that is measured/the outcome Nuisance variables: Extraneous (usually uncontrollable) variables, can affect the dependent variable. Placebo: treatment designed to appear exactly like a comparison treatment 7

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Experimental Design Characteristics Manipulation of an independent variable Control of all other variables except for the dependent variable Observation of the change in the dependent variable  Control Random selection of sample and random assignment to groups Matching of subjects between groups or grouping of subjects based on a nuisance variable Inclusion of a nuisance variable as a treatment variable Statistical removal of a nuisance variable through analysis of covariance 8

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Pre-Experimental Designs Little or no control of extraneous nui­sance variables-the outcome variable is measured, the treatment is given, and then the outcome variable is measured again to see if any change occurred (pre-test/post-test design) Weakness -poor control of nuisance variables 9

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Pre-Experimental Designs 10

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Quasi-experimental Lack full control of all variables-example case control studies Weakness: -Randomization is lacking 11

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Quasi-experimental 12

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC True Experimental Full control of variables by one or more of the methods More control compared to pre and quasi experimental design 13

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC True Experimental Design 14

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Analysis of Variance Statistical technique that applies to a number of experimental designs Advantage of this design is that more than two groups can be used 15

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Validity Correctness of conclusions- Is X related to Y? Internal validity: The extent to which we are correct in our conclusion concerning the relation of independent and dependent variables. External validity: The extent to which we are correct in generalizing the conclusions from sample results to the population. Threats to Internal Validity History and maturation: impact of passage of time Instrument change: Measurements obtained can be affected by changes in instruments. Mortality: loss of subjects either voluntary or death 16

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Validity Threats to External Validity Population validity: accessible population is not equivalent to the intended target population Hawthorne effect: The awareness of being in a study can alter a subject’s responses or behavior, even in medical patients. Psychosomatic effects are quite real, as evidenced by stress ulcers. similarly, and this limits the ability to generalize beyond the study. Experimenter effect: Investigators can also bias results by consciously or unconsciously conveying expectations

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Non-Experimental Study Designs Do not directly manipulate or control an independent variable Observation-main activity Types: -Retrospective Studies - attempt to reason from a present effect, or consequence in a population back to antecedent causes -Prospective Studies- attempt to reason from a present antecedent or event in a population to future consequences or effects 18

Handbook for Health Care Research, Second Edition Chapter 7 © 2010 Jones and Bartlett Publishers, LLC Non-Experimental Study Designs Types: -Case Studies- provides a description of a single subject, with a treatment or trait -Survey- gathers information on a large group of subjects or units by either written or oral questionnaires -Correlational Studies- provide informa­tion on the presence and strength of a relation between two variables 19