Essential Statistics Chapter 81 Producing Data: Experiments

What We’ll Learn ? u Observation studies versus experiments case study I u Experiments Design: - factors and treatments case study II - do comparative experiments, set control u Double blind, single blind u A matched pair design Essential Statistics Chapter 82

Essential Statistics Chapter 83 How Data are Obtained u Observational Study –Sample surveys are observational studies –Observes individuals and measures variables of interest u Experiment –Deliberately imposes some treatments on individuals in order to observe their responses –Studies whether the treatment causes change in the response.

Essential Statistics Chapter 84 Experiment versus Observational Study Goal: detecting a relationship between explanatory and response variables. u Experiment –create differences in the explanatory variable and examine any resulting changes in the response variable (cause-and-effect conclusion) u Observational Study –observe differences in the explanatory variable and notice any related differences in the response variable (association between variables)

Essential Statistics Chapter 85 Why Not Always Use an Experiment? u Sometimes it is hard or impossible to assign people to receive a specific treatment. u Certain explanatory variables, such as handedness or gender, are inherent traits and cannot be randomly assig ned.

Essential Statistics Chapter 86 Case Study I The Effect of Hypnosis on the Immune System reported in Science News, Sept. 4, 1993, p. 153

Essential Statistics Chapter 87 Case Study I The Effect of Hypnosis on the Immune System Objective: To determine if hypnosis strengthens the disease-fighting capacity of immune cells.

Essential Statistics Chapter 88 Case Study I u 65 college students –33 easily hypnotized –32 not easily hypnotized u white blood cell counts measured u all students viewed a brief video about the immune system

Essential Statistics Chapter 89 Case Study I u Students randomly assigned to one of three conditions –subjects hypnotized, given mental exercise –subjects relaxed in sensory deprivation tank –control group (no treatment)

Essential Statistics Chapter 810 Case Study I u white blood cell counts re-measured after one week u the two white blood cell counts are compared for each group u results –hypnotized group showed larger jump in white blood cells –“easily hypnotized” group showed largest immune enhancement

Essential Statistics Chapter 811 Case Study I The Effect of Hypnosis on the Immune System Is this an experiment or an observational study?

Essential Statistics Chapter 812 Subjects, Factors & Treatments u Subjects –individuals studied in an experiment u Factors –the explanatory variables in an experiment. Factors could be one or more. u Treatment –any specific experimental condition applied to the subjects; if there are several factors, a treatment is a combination of specific values of each factor

Essential Statistics Chapter 813 Case Study II Effects of TV Advertising Rethans, A. J., Swasy, J. L., and Marks, L. J. “Effects of television commercial repetition, receiver knowledge, and commercial length: a test of the two-factor model,” Journal of Marketing Research, Vol. 23 (1986), pp

Essential Statistics Chapter 814 Case Study II Objective: To determine the effects of repeated exposure to an advertising message (may depend on length and how often repeated) Effects of TV Advertising

Essential Statistics Chapter 815 Case Study II u subjects: a certain number of undergraduate students u all subjects viewed a 40-minute television program that included ads for a digital camera

Essential Statistics Chapter 816 Case Study II u some subjects saw a 30-second commercial; others saw a 90-second version u same commercial was shown either 1, 3, or 5 times during the program u there were two factors: length of the commercial (2 values), and number of repetitions (3 values)

Essential Statistics Chapter 817 Case Study II u There are six possible treatments, which is a combination from one value of each factor Factor B: Repetitions 1 time3 times5 times Factor A: Length 30 seconds seconds 456 subjects assigned to Treatment 3 see a 30-second ad five times during the program

Essential Statistics Chapter 818 Case Study II u after viewing, all subjects answered questions about: recall of the ad, their attitude toward the camera, and their intention to purchase it – these were the response variables.

Essential Statistics Chapter 819 Comparative Experiments u Experiments should compare treatments rather than assess the effect of a single treatment in isolation u Comparative design ensures that influence other than the experimental treatments operate equally on all subjects

Essential Statistics Chapter 820 Randomized Comparative Experiments u Compare more than one treatment at a time u Make sure that the comparisons are fair: randomly assign the treatments –each treatment should be applied to similar groups or individuals –Use chance to assign subjects to the treatments Randomization and comparison prevent bias

Essential Statistics Chapter 821 Experiments: Basic Principles u Randomization –to balance out lurking variables across treatments u Placebo –to control for the power of suggestion u Control group –to understand changes not related to the treatment of interest

Essential Statistics Chapter 822 Control Group: Case Study Mozart, Relaxation and Performance on Spatial Tasks (Nature, Oct. 14, 1993, p. 611) u Variables: –Explanatory: Relaxation condition assignment –Response: Stanford-Binet IQ measure u Active treatment: Listening to Mozart u Control groups: –Listening to relaxation tape to lower blood pressure –Silence

Essential Statistics Chapter 823 Double-Blind Experiments u If an experiment is conducted in such a way that neither the subjects nor the investigators working with them know which treatment each subject is receiving, then the experiment is double-blinded –to control response bias (from respondent or experimenter)

Essential Statistics Chapter 824 Double-Blinded: Case Study Quitting Smoking with Nicotine Patches (JAMA, Feb. 23, 1994, pp ) u Variables: –Explanatory: Treatment assignment –Response: Cessation of smoking (yes/no) u Double-blinded –Participants don’t know which patch they received –Nor do those measuring smoking behavior

Essential Statistics Chapter 825 Single-Blinded: Mozart, Relaxation and Performance on Spatial Tasks (Nature, Oct. 14, 1993, p. 611) u Variables: –Explanatory: Relaxation condition assignment –Response: Stanford-Binet IQ measure u Not double-blinded –Participants know their treatment group u Single-blinded –Those measuring the IQ

Essential Statistics Chapter 826 Matched Pairs Design u Pairing –to reduce the effect of variation among the subjects –different from a completely randomized design, where all subjects are allocated at random among all treatments

Essential Statistics Chapter 827 Matched Pairs Design u Compares two treatments u Technique: –choose pairs of subjects that are as closely matched as possible –randomly assign one treatment to one subject and the second treatment to the other subject u Sometimes a “pair” could be a single subject receiving both treatments –randomize the order of the treatments for each subject

Essential Statistics Chapter 828 Statistical Significance u If an experiment (or other study) finds a difference in two (or more) groups, is this difference really important? u If the observed difference is larger than what would be expected just by chance, then it is labeled statistically significant. u Rather than relying solely on the label of statistical significance, also look at the actual results to determine if they are practically important.