Collecting Data Sensibly

Chapter 2

Observation and Experimentation

Prayer Can Lower Blood Pressure

2391 people, 65 years or older were followed for six years. People who attended a religious service once a week and prayed or studied the Bible at least once a day were less likely to have high blood pressure. The researchers concluded that “attending religious services lowers blood pressure.”

Observational StudyObservational study –

the investigators observes characteristics of a subset of the members of one or more existing populations.

Goal: draw conclusions about the corresponding population or about differences between two or more populations

Observational StudyDrawbacks:

impossible to draw cause and effect conclusions because we cannot rule out the possibility that the observed effect is due to some variable other than the factor being studied.

Confounding variable – one that is related to both group membership and the response variable of interest in a research study.

Examples“Heartfelt Thanks to Fido”

Researchers measured the heart’s ability to handle stress in patients who had recovered from a heart attack. They found that it was higher (which is good) for those who owned a dog than those who don’t. So should you run out and get a dog immediately if you have a heart attack? Can you think of any factors that might lead to dog owners having better health in general?

Example #2Studies have shown that people over age 65 who

get a flu shot are les likely tan those who do not to die from flu-related illness during the following year. Can you think of any confounding variables that may have been overlooked in this study?

What do to when you can’t observe?

An educator wonders what would happen to test scores if the required lab time for a chemistry course were increased from twice a week to three times a week?

ExperimentationWhat happens when….? or What is the effect of…..? are

not typically questions that can be answered through observation.

Experiment – the investigator observes how a response variable behaves when the researcher manipulates one or more explanatory variables, called factors.

Goal: to determine the effect of the manipulated factors on the response variable.

Drawback: not reasonable to generalize conclusion about group charateristics to the population

ExampleA survey of affluent American (those with

incomes of $75,000 or more) indicated that 57% of all Americans would rather have more time than more money.

Would it be reasonable to generalize from the sample to say that 57% of all Americans would rather have more time than more money? Explain.