6.4 Cause and Effect
The purpose of all correlations is to test for some causality. If you can show a strong correlation, then you are in a good position to prove some cause and effect relationship.
Types of Causal Relationships
Cause and Effect Relationship: a change in X produces a change in Y.
The height of a dropped object increases its impact speed.
Common Cause Relationship: X causes Y and Z to change in the same way. It should be noted that Y and Z could be completely unrelated.
Revenue from parking fees at the beach and local tomato crop are both influenced by the great weather during the summer.
Reverse Cause and Effect Relationship: The dependent and independent variables are reversed in the process of establishing causality.
Researcher thinks that drinking lots of coffee increases anxiety, but finds that nervous people drink lots of coffee.
Accidental Relationship: A correlation exists between two variables that are completely unrelated.
Positive correlation exists between the number of females enrolled in engineering and the number of ‘reality’ shows on TV.
Presumed Relationship: A correlation that is not accidental, but no cause and effect or common cause is apparent.
Suppose there was a correlation between the level of fitness of a person and the number of action movies they watched.
Ex 1 Classify the relationships in the following situations (p.196)
(a) The rate of a chemical reaction increases with temperature.
(b) Leadership ability has a positive correlation with academic achievement.
(c) The prices of butter and motorcycles have a strong positive correlation.
(d) Sales of cellular telephones had a strong negative correlation with ozone levels in the atmosphere over the decade.
(e) Traffic congestion has a strong correlation with the number of urban expressways.
Determining the nature of a relationship can be complicated by extraneous (or external) variables. To reduce the effect of extraneous variables, researchers often compare an experimental group to a control group. The two groups should be as similar a sample as possible so that the extraneous variables have the same effect on both groups. When you change the independent variable for the experimental group, any change observed in the dependent variable for the experimental group (but not in the control group) can be attributed to the change.
Ex 2 A medical researcher wants to test a new drug believed to help smokers quit. Fifty people who want to quit smoking volunteer for the study. The researcher carefully divides the volunteers into two groups, each with an equal number of moderate and heavy smokers. One group is given nicotine patches with the new drug, while the second group is given ordinary nicotine patches. Fourteen people in the first group quit smoking completely, as do nine people in the second group. (p.198)
(a) Identify the control group, the experimental group, the dependent variable and the independent variable.
(b) Can the researcher conclude that the new drug is effective?
(c) What extraneous variables exist in this study?
(d) What further study should the researcher do?
p. 199# 1-5, 8, 11, 14