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4.3 Experiments & Inference HW: Pg 260-262 E23, E25-27, E29, E31.

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Presentation on theme: "4.3 Experiments & Inference HW: Pg 260-262 E23, E25-27, E29, E31."— Presentation transcript:

1 4.3 Experiments & Inference HW: Pg E23, E25-27, E29, E31

2 Cause and Effect Treatments – conditions being compared by an experiment Response – outcome variable Reasoning options: i)A causes B ii)B causes A iii)Lurking variable is responsible

3 Pg 245 D20 Plot Kelly’s results in a dot plot using different symbols for hampters raised in the two different days. Do you think the evidence supports a conclusion that the number of daylight hours causes a difference in enzyme concentration? The difference in the treatments supports Kelly’s hypothesis.

4 Pg 245 D21 Kelly has shown that hamsters raised in less daylight have higher enzyme concentrations that hamsters raised with more daylight. Has Kelly convinced us that that less daylight causes an increase in the enzyme concentration? Almost. 1) all hamsters with higher concentrations of this enzyme happened to get assigned to the short-day group 2) day length caused the enzyme concentrations to change

5 Confounding in Observational Studies Confounded – when the effects of two possible influences are impossible to separate Ex: Determining whether method A or B teaches children to read faster. Boys get treatment A and girls get treatment B. Cannot tell if differences are sex or treatment. Observational Study – no treatments are assigned to units; conditions are already built into the units being studied. Less desirable than experiments Clinical Trial – randomized comparative experiments

6 Pg D22 Suppose surgeons had examined infants without respiratory problems and found that their thymus generally was small A) Have they now proved that a large thymus causes respiratory problems in children? If so, why? If not, what is another possible explanation? No, it might be that respiratory problems caused the thymus B) Design an experiment to determine whether removal of the thymus helps children with respiratory problems. For an experiment, randomly divide the children w/ respiratory problems into two groups: one that gets surgery and one that does not. Treat them the same otherwise and record how many survived each treatment

7 Pg 248 D23 What variables might be confounded with amount of exercise in observational studies on the association of lack of exercise with cancer? Older people are more likely to get cancer than younger ones. If older people are more likely to have sedentary jobs, then it is unknown if it’s age or level of activity at their jobs. Exercise and age would be confounded.

8 Pg 248 D24 What is the main difference between an experiment and an observational study? In an experiment, treatments are assigned to units. In an observational study, conditions you want to compare come already attached to units

9 Factors and Levels Factors – explanatory variables, usually categorical, in a randomized experiment or an observational study. Level – one of the values or categories making up a factor

10 Importance of Randomizing Confounding is the main threat to making reliable inferences about cause. Randomizing is the best defense! Chance or the treatments will be only two possible causes for a difference in responses if treatments are assigned at random. The cause of difference is the treatment, if the probability that chance alone caused the difference in responses.

11 A control or Comparison Group Is Vital Placebo effect – phenomenon that people do better when they believe they are receiving a special treatment Placebo – nontreatment that mimics treatment except for the crucial component. Aka Fake treatment Control Group - group used for comparison; often receives placebo Comparison Group – receives a standard treatment Treatment Group – group given the actual treatment Ex: The Drug Blind – only patients do not know which group they are. Double-blind – both doctor & patient are “kept in the dark.”

12 Pg 252 D25 Why is blinding or double-blinding important in an experiment? Experiments should be blind to curb placebo effect from treatment group. Experiments should be double-blind so that the evaluators do not influence the experiment.

13 Pg 252 D26 A report about a new study to test the effectiveness of the herb St. John’s wort to treat depression says, “The subjects will receive St. John’s wort, an antidepressant drug, or a placebo for a least eight weeks and as long as six months.” Describe how you would design this study to compare the effects of the three treatments. Assign subjects who suffer from depression at random to three groups, one to receive St. John’s wort, one to receive an antidepressant drug and one to receive a placebo. Treat them exactly the same. After 8 weeks, have a doctor evaluate.

14 Pg 252 D27 How would you design an experiment to verify that a placebo effect exists for people who think they are being treated for pain? Assign volunteers randomly to one of the two groups, one receives nothing and one to receive a placebo. Those receiving a placebo are told they are getting a real treatment. Experiment cannot be complete blind because the point is for the subjects to know whether they are getting a treatment or not. Compare results of two groups.

15 Experimental Units and Replication Experimental units – units on which treatments are randomly assigned Replication – random assignment of the same treatment to different units The more good observations, the more faith you can have in conclusions

16 Characteristics of a Well – Designed Experiment Compare - Treatment group is compared to a control group or two or more treatment groups are compared to each other Randomize – treatments are randomly assigned to the experimental units Replicate – each treatment is randomized to enough experimental units to provide adequate assessment of how much the responses from the same treatment vary.

17 Pg 255 D28 In the teaching-methods study (pg 252), why does it seem reasonable that the experimental unit should be the classroom? Provide an example where the experimental unit might be the school? Effectiveness of a method for teaching reading high depnds on the teacher and the classroom atmosphere. The reading scores of the children in the classroom are not independent. It would be difficult to randomly assign different teaching methods to different students in the same classroom. Experimental units receive randomization Needs several schools since that is the exp. unit.

18 Pg 256 D30 Why do you have more faith in your conclusions if your experiment has many replications? As sample size gets larger, the percentage of successes in the sample tends to approach that percentage of success in the population. In an experiment, a large number of subjects tend to ensure the ones in each treatment group are more representative of the whole group designated for study in the experiment.

19 HW: Pg E23, E25- 27, E29, E31

20 CW: Pg P16-18, P21, P24, P26, P27


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