Copyright ©2011 Brooks/Cole, Cengage Learning Gathering Useful Data for Examining Relationships Observation VS Experiment Chapter 6 1

Copyright ©2011 Brooks/Cole, Cengage Learning 2 Replication Replication in an experiment: More than more than one experimental unit is assigned to each treatment condition. Large enough to large enough to provide suitably accurate estimates. If too small then difficult to rule out natural chance variation as reason for any observed differences. Replication in science: A single experiment rarely provides sufficient evidence for anything, so it is important to have independent researchers try to reproduce findings.

Copyright ©2011 Brooks/Cole, Cengage Learning 3 Matched-Pair Designs and Blocking Matched-Pair Designs Use either two matched individuals or same individual receives each of two treatments. Special case of a block design. Important to randomize order of two treatments and use blinding if possible. Block Designs Experimental units divided into homogeneous groups called blocks, each treatment randomly assigned to one or more units in each block. If blocks = individuals and units = repeated time periods in which receive varying treatments; called repeated-measures designs.

-Read the whole case -Use Statistics Terminologies such as explanatory variables, randomized, double- blind…… -Draw the concept map -Present the case to the class. 4 Read and Present Case Study 6.2 and 6.3 (P.197 and P.201)

Copyright ©2011 Brooks/Cole, Cengage Learning 5 6.3Designing a Good Observational Study Disadvantage: more difficult to try to establish cause-and-effect links. Advantage: more likely to measure participants in their natural setting.

1.Types: Retrospective/Prospective/Case- Control Studies 2.Similarities 3.Difference 4.Pros and Cons 6 Compare and Contrast the different types of Observational Study Draw a table or concept map to present the following:

Copyright ©2011 Brooks/Cole, Cengage Learning 7 Types of Observational Studies Retrospective: Data are from the past. Participants are asked to recall past events. Prospective: Participants are followed into the future and events are recorded. Case-Control Studies: “Cases” who have a particular attribute or condition are compared to “controls” who do not, to see how they differ on an explanatory variable of interest. Advantages: Efficiency and Reduction of Potential Confounding Variables through careful choice of “controls”.

Copyright ©2011 Brooks/Cole, Cengage Learning 8 Case Study 6.4 Baldness and Heart Attacks Case-control study cases = men admitted to hospital with heart attack controls = men admitted for other reasons. Explanatory variable: heart attack status (yes or no) Response variable: degree of baldness “Men with typical male pattern baldness … are anywhere from 30 to 300 percent more likely to suffer a heart attack than men with little or no hair loss at all.” Newsweek, March 9, 1993, p. 62

9 What kind of difficulties and disaster you would expect you will experience when you do your observational study or experiment?

Copyright ©2011 Brooks/Cole, Cengage Learning Difficulties and Disasters in Experiments and Observational Studies Confounding Variables and the Implication of Causation in Observational Studies Common media mistake = reporting cause-and-effect relationship based on an observational study. Difficult to separate role of confounding variables from role of explanatory variables in producing the outcome variable if randomization is not used. Extending Results Inappropriately Many studies use convenience samples or volunteers. Need to assess if the results can be extended to any larger group for the question(s) of interest.

Copyright ©2011 Brooks/Cole, Cengage Learning Difficulties and Disasters in Experiments and Observational Studies Interacting Variables A second explanatory variable can interact with the principle explanatory variable in its relationship with the response variable. Results should be reported taking the interaction into account. Example: Interaction in Case Study 6.3 The difference between the nicotine and placebo patches is greater when there are no smokers in the home than when there are smokers in the home.

Copyright ©2011 Brooks/Cole, Cengage Learning Difficulties and Disasters in Experiments and Observational Studies Hawthorne and Experimenter Bias Hawthorne effect = participants in an experiment respond differently than they otherwise would, just because they are in the experiment. Many treatments have higher success rate in clinical trials than in actual practice. Experimenter effects = recording data to match desired outcome, treating subjects differently, etc. Most overcome by blinding and control groups.

Copyright ©2011 Brooks/Cole, Cengage Learning Difficulties and Disasters in Experiments and Observational Studies Ecological Validity and Generalizability When variables have been removed from their natural setting and are measured in the laboratory or in some other artificial setting, the results may not reflect the impact of the variable in the real world. Example 6.7 Real Smokers with a Desire to Quit Case Study 6.3: Ensured ecological validity and generalizability by using participants around the country of wide range of ages, and recorded many other variables and checked that they were not related to the patch assignment or the response variable.

Copyright ©2011 Brooks/Cole, Cengage Learning Difficulties and Disasters in Experiments and Observational Studies Using the Past as a Source of Data Can be a problem in retrospective observational studies. Try to use authoritative sources such as medical records rather than rely on memory. If possible, use prospective observational studies.

Homework Assignment: Chapter 6 – Exercise 6.33 and 6.50 Reading: Chapter 6 – p