F25-Design-SampleSize

F25-Design-SampleSize - PubH 7405: REGRESSION ANALYSIS...

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PubH 7405: REGRESSION ANALYSIS DESIGN ISSUES: VALIDITY & SAMPLE SIZE
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OF CLINICAL RESEARCH We form and/or evaluate a research or research project from/on two different angles or parts: the anatomy and the physiology of research; just like the hardware and software to run a computer operation.
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THE ANATOMY PART From the anatomy of the research, one can describe/see what it’s made of ; this includes the tangible elements of the study plan : research question, design , subjects, measurements , sample size calculation , etc… The goal is to create these elements in a form that will make the project feasible , efficient , and cost-effective .
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THE PHISIOLOGY PART From the physiology of the research, one can describe/see how it works ; first about what happened in the study sample and then about how study findings generalized to people outside the study . The goal is to minimize the errors that threaten conclusions based on these inferences.
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Very briefly, designed experiments are conducted to demonstrate a cause-and-effect relation between one or more explanatory factors (or predictors) and a response variable. The demonstration of a cause-and-effect relationship is accomplished by altering the level or levels of the explanatory factors and observing the effect of the changes (i.e. designed values of predictors X’s) on the response variable Y. There is a good reason that designed experiments are often used as “ comparative in natures. Why?
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Two major levels of inferences are involved in interpreting a study The first level concerns Internal validity ; the degree to which the investigator draws the correct conclusions about what actually happened in the study. The second level concerns External Validity (also referred to as generalizability or inference ); the degree to which these conclusions could be appropriately applied to people and events outside the study.
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Truth in The Universe Truth in The Study Findings in The Study Research Question Study Plan Study Data External Validity Internal Validity
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With the goal of maximizing the validity of the inferences, the investigator reverses the process: (i) designs a study plan in which the choice of the research question , the subjects , and the measurements enhances the External Validity , (ii) is conducive to implementation with a high degree on Internal Validity . That is to focus on the External Validity first ( Design ) then Internal Validity ( Implementation ).
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THE BASIC ISSUE Most of the times, inexperienced researchers mistakenly act like there is an identifiable, existent parent population or populations of subjects. We act as if the sample or samples is/are obtained from the parent population or populations according to a carefully defined technical procedure called random sampling. And we simply compare population means.
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This note was uploaded on 11/21/2011 for the course PUBH 7405 taught by Professor Staff during the Fall '08 term at Minnesota.

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F25-Design-SampleSize - PubH 7405: REGRESSION ANALYSIS...

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