Vocab Review Observational Study: Passively observe, record, measure, question, but you don’t personally affect the situation Experiments: Active data production, impose a treatment to see what happens A change in the Explanatory Variable causes the Response Variable to change Explanatory Variable = Independent Variable Response Variable = Dependent Variable A lurking variable is a variable that affects things but wasn’t measured

New Experiment Vocabulary Individuals studied in an experiment are often called subjects. A treatment is any specific experimental condition applied to the subjects. A clinical trial is a trial on actual human patients

Designing an Experiment For any experiment you want to have 2 groups: the control & the experimental group In an experiment there should be only 1 difference between your control group and the experimental group. That difference is the treatment; it is the explanatory/independent variable The effect that you measure is the response/dependent variable

Steps in Designing an Experiment 1) Collect a group of subjects 2) Randomly split your subjects into 2 groups. The groups should be ideally indistinguishable: same gender ratios, same racial makeup, same percentage smoke, have cancer, etc. Group A Group of Subjects Group B

Steps in Designing an Experiment 1) Collect a group of subjects 2) Randomly split your subjects into 2 groups. 3) Apply a treatment to 1 group and not the other. This treatment is the explanatory variable Group A Control Group of Subjects Group B Treatment

Steps in Designing an Experiment 1) Collect a group of subjects 2) Randomly split your subjects into 2 groups. 3) Apply a treatment to 1 group and not the other. 4) Compare results. The variable you measure is the response variable Group A Control Group of Subjects Compare Results Group B Treatment

Idea of an Experiment If there is only one difference between the two groups, then that difference must be the cause of any difference in the end result. Group A Control Group of Subjects Compare Results Group B Treatment

Variable Vocab Review Causation X Causes Y Confounding X & Z Cause Y Happens when there are 2+ differences between control & experimental groups Common Response Z Causes X & Y

3 Basic Principles of a Good Experiment Control the effects of lurking variables on the response variable. Use a control so lurking variables cause the same impact on both groups and therefore can’t cause a difference. Randomize use impersonal chance to pick subjects and to split them into groups Use enough subjects to minimize variation & random sampling error.

Example 1: Question of Interest Most people know that drunk driving is dangerous. Even so, some drivers are cited multiple times for driving while intoxicated. Is there anything that can be done to change their behavior to stop repeat offenders? An experiment was designed to help answer this question.

Example 1: Grouping The subjects were 300 people convicted of drunk driving 3 times in 1 year. The treatments, imposed after the third conviction, were… Fine & Suspended Jail Sentence Fine, Suspended Jail Sentence & Mandatory Attendance to an Alcoholic Clinic Fine, Suspended Jail Sentence & Mandatory Participant in Alcoholics Anonymous Which is the control? Fine & Suspended Sentence

Example 1: The Dilemma If you wait until you have 300 convictions, then you adding a lurking variable, time since conviction. It is important that the treatment is applied immediately after the conviction. When each conviction happens, is it better to assign treatments in the order 1, 2, 3, 1, 2, 3, etc. to ensure that each sample is the same size or should you randomly assign each person at the time of conviction to promote randomness, but then the 3 groups may be slightly different sizes? Randomize

Placebo The mind is more powerful than we understand. If you believe something, it is more likely to happen. People who believe medicine helps are more likely to recover from a disease while on medication People who are afraid of side effects are more likely to have side effects from medication A placebo is a fake treatment so subjects in the control group don’t know they are in the control group.

Placebo Knowledge of what group you are is a variable. That can be a second difference between the experimental & control. We use placebos to trick people. They take sugar pills, they receive injections that contain no actual medicine

Example 2 “Gastric freezing” is a clever treatment for stomach ulcers. The patient swallows a deflated balloon with tubes attached; then a refrigerated liquid is pumped through the balloon for an hour. The idea is that cooling the stomach will reduce its production of acid and so relieve the ulcers. How would you test this without the subject knowing what group they are in? Have the control swallow a balloon and then pump room temperature liquid through it. The temperature of the liquid is the only variable difference!

Example 3 Let’s say there was a tragic accident in this room and you all lost your sense of smell. I have new drug that we think will bring back your smell. If I give it to all of you and 80% get your smell back, is that proof the drug worked?  No, how do I know you wouldn’t have gotten better on your own without the drug? So I break you into two groups and give the drug to half and don’t to half. The half getting the drug, 80% got their smell back while only 50% of those who had no treatment. Is that proof? No, placebo effect. What if I gave half a real drug and half a fake drug? Better, but what if you read my expression and figured out which group you were in that way?

Placebo A blind study is when the subjects don’t know what group they are in. The doctor gives 1 group the medicine & 1 group the placebo A double blind study is when the subjects and the doctor doesn’t know what group they are in. Instead there is a third party running the experiment who tells the doctor give pill A to him and pill B to her.