Experiments vs. Observational Studies

  • Ovservational Study

    • Observe individuals

    • Measure variables

    • Do NOT influence the response

      • Has global warming effected penguin mating behavior
  • Experiment

    • Do something to your individuals

    • Observe/measure response

      • Does housing penguins in warmer environments effect mating behavior?

Placebo Effect

  • The phenomenon where patients get better because they expect the treatment to work.

  • Many statistical studies involve testing the effectiveness of drugs. A placebo looks identical to the actual drug but contains no active ingredient and so has no real physical effect.

  • Humans want to be helped by the medication that is administered to them. If they think they are receiving a drug to help their condition, they tend to improve even if it turns out that the drug is a placebo.

Characteristics of a Well-Designed and Well-Conducted Experiment

  • Control

    • The effect of lurking variables, most often by comparing treatments

    • Example: a "Control group" in a drug study to eliminate the "confounding effects" of environment or the placebo effect

  • Replicate

    • Each treatment on many units to reduce chance variation

    • Example: do the mouse study many times

  • Randomize

    • Use probability (chance) to assign experimental units to treatments

    • May be the most important!!

    • Because it allows us to say the different treatment groups start out similar

Completely Randomized Design

  • If all the experimental units (subjects of the experiment) are randomly assigned to either the control group or to the treatment group, then the experiment has a completely randomized design.

    Random Selection Subiects Treatment Group A Treatment Group B
Compare Results Treatment Group C Control Group

  • Randomize by assigning each subject a number and then generating it to choose treatment groups

Block Randomization

  • Placing subjects into groups of similar individuals. The random assignments into treatment groups is carried out separately within each block (think stratified random sample)

    Random 300 Male Subjects 500 Subjects Female 200 Female Subiects
Treatment Group A 75 Male Subiects Treatment Group B 75 Male Subiects
Treatment Group C 75 Male Subiects Control Group 75 Male Subiects
Treatment Group A 50 Female Treatment Group B 50 Female Subjects
Treatment Group C 50 Female Subiects Control Group 50 Female SubiectS
Compare Results Compare Results

Matched Pairs Design

  • Subjects are matched into pairs and get different treatments

  • Matched pairs are more similar than random unmatched subjects

  • Randomizing the rest of the experiment is still important!!!

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Experimental Set Up

  • Treatment Imposed = Independent Variable = Factors

  • Experimental Units = Subjects

  • Response Variable Observed = Dependent Variable

    Treatment imposed aka 'independent variable' aka 'factors'
experimental units aka 'subjects' Response rough strain (nonvirulent)
smooth strain ( virulent ) heat-killed smooth strain rough strain &
heat-killed smooth strain variable observed aka 'dependent mouse lives
mouse dies mouse lives mouse dies variable'

Double-Blind Experiment

SINGLE BLIND Patients Con t Test Doctor

DOUBLE BLIND Patients Con t Test Doctor

  • In a double-blind experiment, neither the subjects nor the researchers know to which group, treatment, or control, subjects have been assigned. If a researcher knows that a subject is in the control group, they do not expect a treatment effect, and their measurement of a response might be understated. If a researcher knows that a subject is in the treatment group, they might overstate a response simply because they expect it.

  • An experiment might also be single-blind. In this case, only one of the participants, either the subjects or the researchers, knows to which group the subjects have been assigned.

  • Avoids unconscious bias

Generalizability of Results

  • To determine if our data is "statically significant"

    • i.e. is an observed effect so large that it would rarely occur by chance
  • If we designed and conducted our experiment well, we can generalize these results to the population!

Practice Questions

  • Control groups are used in experiments in order to

    1. Control the effects of outside variables on the outcome

    2. Control the subjects of a study to ensure that all participate equally

    3. Guarantee that someone other than the investigators, who have a vested interest in the outcome, controls how the experiment is conducted

    4. Achieve a proper and uniform level of randomization

    Answer: a

  • Angiotensin-converting enzyme (ACE) inhibitors are used to treat high blood pressure. We want to conduct an experiment to see if a new blood pressure drug works even better than ACE inhibitors. Design a completely randomized experiment to test this.

    60 30ΎΖ

  • In conducting an experiment to see if a new blood pressure drug works even better than ACE inhibitors. We learn that men and women may react differently to common cardiovascular drug treatments. Design a randomized experiment to test this with your new information on gender.

    • We will conduct a randomized blocked experiment, blocking on gender.

    ゝ 3 マ ( , ~ 7 わ d4

  • The Community Intervention Trial for Smoking Cessation (COMMIT) asked whether a community-wide advertising campaign would reduce smoking. The researchers located 11 pairs of communities that were similar in location, size, economic status, and so on. One community in each pair participated in the advertising campaign and the other did not. This is

    1. an observational study

    2. a matched pairs experiment

    3. a completely randomized experiment

    4. a block design

    Answer: b

  • A study of cell phones and the risk of brain cancer looked at a group of 469 people who have brain cancer. The investigators matched each cancer patient with a person of the same sex, age, and race who did not have brain cancer, then asked about the use of cell phones. This is

    1. an observational study

    2. an uncontrolled experiment

    3. a randomized comparative experiment

    4. a matched pairs experiment

    5. a survey

    Answer: a

  • A fitness instructor wants to test the effectiveness of a performance-enhancing herbal supplement. Design an experiment to test this supplement

    • Double blind, (placebo controlled), matched pairs experiment:

    • Match subjects based on performance in a fitness test and gender

    • Randomize who in the pair gets the new supplement and who gets the old supplement / placebo.

    • Give both in the same packaging, making sure the subject doesn't know the group and person measuring fitness doesn't know the group either.

  • A researcher believes that students may do better on a test when taken in the same classroom where the material was learned. To test this theory she plans to present a lecture and then give students a multiple choice quiz on the material. She knows there is a lot of variability in the students' academic ability. Design a study to test her hypothesis.

    • Block on academic ability


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