Correlational versus Experimental

Study on Alcohol and Speech Researchers were quite interested in the effect of alcohol on speech. They obtained four samples of 60 adults - one from the west coast states, one from the southern states, one from a large Midwestern city and the area around it, and one from the eastern seaboard states. The 240 participants were run one at a time. The participants in each region of the country were randomly divided into two groups. In group 1, each participant was asked to read a few paragraphs into a microphone. Then they were given two alcohol drinks of their choice. After 20 minutes blood alcohol level was measured using a recently calibrated breathalyzer. If it was not over.1%, a third drink was given to the participant. After participant's blood alcohol level was over.1%, the subjects were again asked to read the paragraphs into the microphone. Group 2 was treated similarly except they drank the alcohol before their first reading, then waited until their blood alcohol level dropped to zero before making their second reading. After all data was collected, the researchers used computer equipment to carefully compare the participants' speech in the two conditions. One important measure of participants' speech was the time it took them to say words such as "Valdez." The researchers found that participants took 30% longer to say words under the influence of alcohol. Can we say that an elevated blood alcohol level causes speech to be slower? AnswerAnswer

Does elevated blood alcohol level cause speech to be slower? This study provides evidence that it does. It is an experiment that includes manipulation of the amount of blood alcohol so the term cause can be used. The number of subjects is not too small. There is no reason to believe that the measurement of blood alcohol level or time to say words is biased given that both were measured using machines. Furthermore, by splitting the sample into groups and varying when they drank and spoke, the researchers minimized such confounding variables as fatigue. This is a well done study that leads to the conclusion that elevated blood alcohol level causes speech to be slower.

Study on Smoking and Lung Cancer A young researcher wanted to do the definitive study on smoking and lung cancer. On his 28th birthday, he received a grant to study adults in the United States and Canada who were between the ages of 45 and 55. He randomly selected 100,000 subjects from census information. All of the subjects were sent questionnaires (and his assistants phoned or visited in person if the subject did not respond). The questionnaire included a number of questions about how much the subject had smoked in their lives. Forty years later, the researcher's large team of assistants searched records to determine cause of the death of the adults in the study. For most of the subjects, the cause of death was listed on the death certificate. For those subjects that had ambiguous causes stated on the death certificate, the assistants searched death notices in newspaper articles, contacted doctors, relatives, hospitals, etc. They could not obtain records for 23 missing persons and 52 of the subjects were still alive. Analysis of the 99,925 records showed that smokers were 42 times more likely to die of lung cancer than non-smokers if they smoked one pack or more of cigarettes a day. The researcher concluded that this study clearly shows that lung cancer is caused by smoking. Can we say from this study that smoking causes lung cancer? AnswerAnswer

smoking causes lung cancer? This study does not provide evidence of causality! In spite of all the things the researcher did well (perhaps even overdid), there is no manipulation of the amount of smoking and therefore there is no legitimate way to conclude causality. The research measured it by asking people how much they smoked. There could be bias in these answers, but it is unlikely that bias can explain the rate of lung cancer. The problem is that some other factor - perhaps a genetic one - underlies both the likelihood of smoking and the likelihood of having lung cancer. Without controlled manipulation of the amount of smoking, it is not possible to rule out these alternative explanations of the data.

A newspaper headline reads, "Binge Drinkers Get Lower College Grades." What kind of study can lead to this conclusion? People remember concrete words (e.g., dog, elephant, or chair) better than abstract ones (love, freedom, or desire). What kind of study leads to this conclusion? People who were abused as children are more likely than others to become child abusers. What kind of study leads to this conclusion? A health magazine reports that depressed people who continue medication for at least six months are less likely to relapse than are people who take medication for less than six months. What kind of study leads to this conclusion?