Chapter 8 Control

Supplemental Information (Not in Textbook) Control in experiments maximizes internal validity High internal validity of research = ability to infer CAUSAL associations Most correlations do not describe a causal relationship Examples………..

Control by selection “Attending a private schools correlates with better performance on standardized tests (versus attending public schools). “ As a result of this observation, many people believe that sending children to private schools will help increase test scores.

Control by selection One of the problems with this inference is that there are other variables that could influence both the kind of school a kid attends and his or her test scores. For example, the financial status of the family is a possible confound. “quality” of school test scores + + financial status

Correlation and Causation Amount of ice cream sold on the beach correlates with the number of drownings What are some explanations? Group discussions

Correlation and Causation Amount of ice cream sold on the beach correlates with the number of drownings What are some explanations? Eating ice cream causes drownings (?) People console themselves with ice cream when people drown (?)\ Third factor----increasing summer temperatue increases beach populations and hence, food sales (ice cream) and drownings

Correlation and Causation Increases in number of people living in Louisiana correlates with increasing loss of land mass due to sinking More people causes more weight atop the land and hence, more sinking of land in LA (?)

Correlation and Causation Increases in number of people living in Louisiana correlates with increasing loss of land mass due to sinking Coincidental association; geological forces cause sinking, with/without people atop the land.

Correlation and Causation In 1974, a decrease in national speed limit to 55 is correlated with decreases in highway death rates the following year A causal relationship here?

Correlation and Causation In 1974, a decrease in national speed limit to 55 is correlated with decreases in highway death rates the following year A causal relationship here? No, middle eastern countries limited sales of oil to the US (an oil embargo). Price of gas increased, people drove less. Gas became less available (spot shortages) and people drove less. Fewer total drivers on the road per mile probably caused the decrease in highway deaths in 1975

What is Simpson’s Paradox? This will be a question reflected in Exam 2

Supplement to Textbook Social policy-makers, political leaders, science groups declare “causal associations” based on preponderance of evidence from a large body of less-than-optimal science and indirect evidence Notion of Evaluating “Converging Evidence” to decide Causality

Example: Does Smoking Cause Lung Cancer? Examples: Lung Cancer and Smoking Global Warming

Example: Does Smoking Cause Lung Cancer? Examples: Lung Cancer and Smoking Optimal experimental design regarding smoking and lung cancer cannot be conducted for practical and ethical reasons What would such a study look like? Discuss, explain

Example: Does Smoking Cause Lung Cancer? Criteria used by Surgeon General’s commission on smoking in 1964 Strength of the association Example Consistently, we find that smokers are many times more likely to develop lung cancer than nonsmokers Consistency of the association Example: diverse correlational studies show the same association

Example: Does Smoking Cause Lung Cancer? Criteria used by Surgeon General’s commission on smoking in 1964 Specificity of the association Example: speculations about other possible causal factors prove to be in error when any of these other causal factors are studied and controlled for e.g., living in urban area with polluted air Example: lung cancer rates can be predicted from smoking data and smoking is found to predict lung cancer

Example: Does Smoking Cause Lung Cancer? Criteria used by Surgeon General’s commission on smoking in 1964 Temporal relationship of the association Cancer always follows smoking behavior, not vice versa Once people in a population stop smoking, rates of lung cancer subsequently decrease over time

Example: Does Smoking Cause Lung Cancer? Criteria used by Surgeon General’s commission on smoking in 1964 Coherence of the association Example: all available animal and human studies suggest the association exists between smoking and cancer in animals and humans; scientific findings about the nature of carcinogens and causes of cancer support the association

Interpreting Correlations (Supplement to Textbook) Correlation between teachers’ salaries and the amount of liquor sold Increased drinking caused by increasing salaries among teachers?

Control Ways to rule out threats to validity Two basic meanings: Standard for comparison Minimizing variability

How to Increase Experimental Control Use a lab, if possible Consider the experimental setting Instrumentation for measuring responses Make nuisance variables into IVs Statistical control Replicate your findings

Basically 2 Types of Experiments Within-Subjects Experiment Each subject experiences ALL experimental conditions. Between-Subjects Experiment Each subject experiences only ONE experimental condition

Control in a Within-Subjects Design Each subject is used as their OWN control. Control Condition Occasion when the subject DOESN’T get the IV. Experimental Condition The subject gets the IV. The participant’s scores (DV) on the control condition are compared to those from the experimental condition.

Control in a Between-Subjects Design Experimental Group Subjects who get the IV Control Group Subjects who DON’T get the IV Random Assignment Every subject has an equal and independent chance to be in a group. Matching Making sure that the experimental and control groups are equal on one or more variables BEFORE the experiment starts.