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Experiment Subjecting the sample to a controlled treatment. The objects on which the treatment is imposed on are called subjects. The response variable measures an outcome or result of a study. Explanatory variables explain or cause a change in the response variable. These determine the treatments imposed on the subjects.
An experiment with more than 2 treatments: Studying the Durability of Fabric under repeated washings. • What are the explanatory variables? • What is the response variable? • What are the subjects?
Completely Randomized Design • Randomly assign a treatment to each experimental unit • The number of units assigned to each treatment is as equal as possible • Compare the results of the response variable to see if any of the treatments made any significant difference.
Is weight training good for children? If so, is it better for them to lift heavy weights for a few repetitions or moderate weights a larger number of times? • Subjects • Random assignment • Comparison groups(explanatory) • Response 14--Heavy load group Measure muscular strength & endurance Compare between groups 43 volunteers Randomly assigned to 3 groups 15--Moderate load group 14--Control group – no weights
3 Principles of Good Experimental Design Randomization--randomly assigning subjects to treatment groups to reduce the likelihood that the results will be affected by confounding variables and other sources of bias Control— comparing two or more treatments Replication--consistency to many subjects, using enough subjects to reduce chance variation in the results
Each subject draws out of a bag a colored chip - each treatment is a different color Each treatment is assigned a number or set of numbers - then a random number is generated for each subject to assign the treatment Each subject rolls a die - odds go to one treatment, evens to the other Each subject flips a coin - heads go one treatment, tails to the other Ways to Randomize
What if we didn’t randomize? • Suppose we took volunteers to participate in a study to reduce stress at the workplace • For six weeks, one group took a yoga class, and the other group took a kickboxing class. The volunteers chose the class they would participate in. • If the participants in the yoga classes significantly reduced their stress level, could we say that it was due to the yoga class? • What might be a confounding variable? • Friendship; maybe all the people that took yoga are friends, but the people who took kickboxing are just acquaintances. The yoga group’s lower stress level could be from the friendship, not necessarily the type of activity. • Confounding is not the same as variation; if both groups had some friends and some acquaintances in it that wouldn’t be a problem
Control group--receives standard/traditional treatment OR aPlacebo -receives no treatment but subjects believe they are receiving treatment Control Groups & Blindness Single Blind:subjects don’t know which treatment they receive Double Blind:subjects and evaluators are “blind”; only the researcher has the “key”
From the Ask Marilyn Column in Parade Magazine • Doug Johnson of Denham Springs Louisiana writes:Marilyn: If you wished to test the effects of a placebo, what would you give the control group? Marilyn responds:Ha, ha—very funny, Doug. But to answer your question, you’d give them the real thing! The difference is that you’d be interested in the effectiveness of the placebo instead of the drug. Placebos have even been tested against each other to determine which is more effective. In one study, sham acupuncture was pitted against dummy pills for chronic arm pain. The acupuncture worked better. Also, before the study began, subjects were advised of possible side effects. About 25 percent of the false-acupuncture patients and 31 percent of the fake-pill patients reported experiencing them!
Designing a good experiment will: • Reduce the effect of any confounding variables, as they should be evenly spread out among the treatment groups. • Determine cause and effect – any statistically significant differences (so large they would rarely occur by chance) in the response variable can be attributed to the treatments
Quitting Smoking w/Nicotine Patches Recruited 240 smokers (volunteers) at Mayo Clinic from 3 large cities Randomly assigned 22-mg nicotine patch or placebo patch for 8 weeks. All attended counseling before, during, and after. Double-blind After 8-wk (1 yr), 46% (27.5%) of nicotine patch group quit smoking and 20% (14.2%) of placebo group quit.
Quitting Smoking w/Nicotine Patches • What are the subjects? • What are the treatments? • What was the explanatory variable? • What was the response variable? • How was randomization applied? • How was control applied? • How was replication applied? • Is this an experiment or an observational study? • How would you summarize the results of this experiment?
Design the experiment: • A baby-food producer claims that her product is superior to that of her leading competitor, in that babies gain weight faster with her product. 30 healthy babies are available for a clinical trial.
Completely Randomized Design • Randomly assign babies to treatments. Have 15 red and 15 blue chips in a bag and draw one for each baby. Reds get her product, blues get the competitor’s product. • Or …Assign evens/odds to new/standard product. Get a random number for each baby, assigning food for each until one group has 15 in it. The rest go to the other group. • Compare the weight gain of the babies after the two month period.
Assignment using Random Number Table • Use the digits below to randomly assign the babies to the two food products. • Read a number for each baby. If it’s odd, the baby gets food A. If it’s even, the baby gets food B. Once 15 babies have one food, the rest get the other. • 15458 56410 54791 25468 03544 65042 • Food A: Babies 1,2,4,6,9,11,13,14,15,17,22,23,27,29,30 Food B: Babies 3,5,7,8,10,12,16,18,19,20,21,24,25,26,28
Ethical Guidelines in a Study • Institutional Review Board: Reviews all studies in advance so that subjects are protected from possible harm
Informed Consent • Given to subjects before data is collected • Nature of the study • Risk of possible harm • What type of questions will be asked • How long the study will take • For who might informed consent be difficult to obtain? • Behavioral studies (“what would you do?” on abc) • People who can’t speak for themselves (infants, mentally ill)
Confidentiality • Individual data is not released; only statistical summaries for groups of subjects are made public • Not the same as Anonymity: names are not known to even the director of the study • Mail response surveys • All studies must have confidentiality, not all need or could have anonymity
Clinical Trials • Used only if there is hope that the subjects will be helped • If a treatment is already proven to work and is safe, it is unethical not to give it • If we don’t know which treatment is better, or we don’t know if a treatment is better than the placebo, it is ethical to give both. • PPD Website: • http://www.ppdi.com/study_volunteers/phase_i_clinic/faq.htm