Surveys knowingly respond

Transcription

1 Observational Studies no intervention no treatment Experimental Studies Experiments intervention or treatment Surveys knowingly respond 1

2 In observational studies or surveys, select randomly so the sample represents the population In experiments, assign treatments randomly to evenly distribute variables that we can't control 2

3 When: 1747 Where: The HM Bark Salisbury Who: James Lind, the ship's surgeon What: Used a controlled experiment on pairs of crew members who "were as similar as I could have them" to develop a cure for scurvy. How: 2 week treatment as follows: 3

4 1 qt cider/day Crew Lind chose 12 subjects with scurvy divided pair 1 pair 2 pair 3 pair 4 pair 5 pair 6 25 drops elixir vitriol (H 2 SO 4 ) 3x/day on empty stomach 1 / 2 pint seawater/day nutmeg sized mixture of garlic, mustard & horseradish 2 spoonfuls vinegar 3X/day Compare. 2 oranges & 1 lemon/day 4

5 Result: The men given citrus fruits recovered dramatically within a week. One returned to duty after 6 days & the other became nurse to the rest. The others experienced some improvement, but nothing was comparable to the citrus fruits, which were proved to be substantially superior to the other treatments. Control Replication Randomization Lind provided strict entry requirements to reduce extraneous variation. The men were paired, which provided replication. From a modern perspective, the main thing that is missing is randomized allocation of subjects to treatments. 5

6 The vocabulary of experiments Experimental Units: Objects we experiment on. Subjects: Human experimental units. Factor = Explanatory/Independent Variable you manipulate Level = value, amount, or subdivision of each Factor (a Factor can have an absent level) Treatment = Each Factor/Level combination, specific experimental condition applied to units ActivStats III Basic Concepts 6

7 Outcome = Response/Dependent Variable (what you measure on units) Single Blind = when either subjects or evaluators don't know which group subjects were assigned to. Double Blind = when neither subjects nor evaluators don't know which group subjects were assigned to. Placebo = treatment (in medical studies) contains no medication, used in "blind" studies. Placebo Effect = when placebo affects response variable 7

8 control group: receives no treatment or a placebo (to control effect of outside variables) this avoids experimental bias (such bias favors some outcome) 8

9 You're now ready to try p. 293 exercises 31, 33, & 35 9

10 Three major principles of experimental design: Control (use units that are similar) Randomization (assign treatments randomly) Replication (experiment on several units) Control any variables you can. Randomize to even out those you can't. Replicate so that patterns in data are noticeable. 10

11 Control potential confounding variables that might affect response variable. Control those by using block design or matched pairs design. examples controlling and comparing gender, health, age, or smoking history Regional tastes, gender, or age age, gender, disabilities, IQ can affect medical research marketing research education research 11

12 Randomize to balance any unexpected or unconsidered variables evenly among all groups! Experimental Units or Subjects Randomly assign group 1 group 2 group 3 group 4 12

13 Replicate to identify patterns. Individual results are just anecdotes. Large numbers of experimental units reduce chance variation. Medications are not tested for safety and effectiveness on just two subjects! McDonald's would not offer a Basil Okra Sundae based on market research of only 9 people. + = 13

14 Because of Control Randomization Differences among groups must be due to treatment or chance Replication 14

15 Principles of Experimental Design 1. Control the effects of lurking variables on the response, most simply by comparing two or more treatments. 2. Randomize use impersonal chance to assign experimental units to treatments. 3. Replicate each treatment on many units to reduce chance variation in the results. An observed effect so large that it would rarely occur by chance is called statistically significant. 15

16 Three Experimental Designs you need to know: 1. Completely Randomized random assignment to treatments 2. Randomized Block first separate into "blocks" with similar characteristics run mini experiments in each block 3. Matched Pairs before & after experiment or other pairing of similar units 16

17 Completely Randomized Does yogurt help prevent the body from absorbing fat? A study randomly assigned subjects to one of 3 groups. One group had no dietary changes, the second group added 2 cups of yogurt to their diet each day, & the third group added 2 ounces of cheese to their diet each day. Researchers collected stool from all subjects throughout the month long study and compared the amount of fat excreted (and therefore not absorbed) for the 3 groups. Experimental Units or Subjects Randomly assign group 1 group 2 group 3 no change yogurt cheese Compare 17

18 You're now ready to try p. 298 exercises 37, 39, & 41 18

19 Randomized Block Researchers are testing a new cholesterol lowering drug on 24 volunteers, 12 men and 21 women. There is reason to believe that the drug may affect men and women differently, so they block on gender. Half the men and half the women are randomly assigned to each of 3 groups. One group gets a placebo, the other 2 groups get 1 of 2 different doses of the drug. 12 men Randomly assign 4 men placebo 4 men 4 men 25 mg Compare 50 mg Experimental Units or Subjects 21 women Block Randomly assign 7 women 7 women 7 women placebo 25 mg Compare 50 mg 19

20 Matched Pairs Textile researchers have developed a new fabric that insulates better than a currently popular fabric used in gloves. However, they want to see if the new fabric wears as well when woven as thin as the current fabric. For 3 winter months, 60 volunteers in Fond du Lac, Wisconsin are given a pair of gloves each. Randomly within each pair, one is made of the new fabric and one of the current fabric. After 3 months, the gloves are examined for wear. Experimental Units or Subjects Randomly assign group 1 group 2 L new R current L current R new Compare 20

21 You're now ready to try p. 303 exercises 43, 45, & 47 21