Surveys, Experiments, and Observational Studies

This information begins on page 23 of the textbook that you may get by

the end of the week.

Experiment vs. Observational Study

• An experiment imposes a treatment on individuals to collect data on their response to the treatment.

• An observational study observes individuals and measures variables without controlling the individuals or their environment in any way.

Experimental or Observational

• A researcher asks students the average number of hours of sleep they get per night and examines whether the amount of sleep affects students’ grades.

• The researcher gathers data without controlling the individuals or applying a treatment.

• The situation is an example of an observational study.

Experimental or Observational

• A park employee wants to know if latex paint is more durable than non-latex paint.

• She paints 50 benches with latex paint, and 50 with non-latex paint.

• The employee applies a treatment (painting benches with latex paint) to some of the individuals (benches). The situation is an experiment.

Controlled Experiment

• In a controlled experiment, two groups are studied under conditions that are identical except for one variable.

• The effects of the treatment are determined by comparing the control group and the treatment group.

Controlled Experiment

Treatment Group

Control Group

Evaluating a Published Report

Milk Fights Cavities

• At Ashland Middle School, fifty randomly chosen students were given milk at lunch every day for a year, and fifty other randomly chosen students were given other beverages.

• At the end of the year, students in the “milk” group had 15% fewer cavities than students in the other group.

Treatment:

• Milk everyday at lunch

Treatment Group:

• 50 randomly chosen students

Control:

• Another 50 randomly chosen students

Randomized Comparative Experiment

• A randomized comparative experiment should be used to gather data whenever feasible because this type of study makes it possible to draw valid cause-and effect conclusions.

• Such experiments are also reliable. That is, they can be repeated and can be expected to produce similar results each time.

Experiment or Observational

• What is the best way to answer the question:

Does listening to an MP3 player with earphones for more than one hour per day affect a person’s hearing?

• Could the treatment harm a participant?

Yes

• Due to ethical considerations you must use an Observational Study

Experiment or Observational

• What is the best way to answer the question:

Do people who consume 1000 milligrams of vitamin C each day as a dietary supplement have lower cholesterol levels than people who do not consume vitamin C supplements?

• Could the treatment harm a participant?

Nope

• So conduct a randomized comparative experiment.

What is the difference in an Observational Study and a Survey?

• Surveys are not comparative

• Surveys draw data from only one group

• So Surveys cannot make conclusions about cause and effect

• It is possible to give the same survey to two or more groups and compare the results – but that is a type of observational study!

Survey, Experiment, or Observational

• A researcher is considering three methods of evaluating two different cold medicines. Tell whether each method is a survey, an experiment or an observational study.

• Method A – Choose 50 people at random. Ask which cold

medicines they have taken in the past, and how effective they were.

• This method is a survey.

Survey, Experiment, or Observational

• A researcher is considering three methods of evaluating two different cold medicines. Tell whether each method is a survey, an experiment or an observational study.

• Method B – Monitor 50 people with colds, and measure the

length of the symptoms for the individuals who choose to take each type of medicine.

• This is an observational study.

Survey, Experiment, or Observational

• A researcher is considering three methods of evaluating two different cold medicines. Tell whether each method is a survey, an experiment or an observational study.

• Method C

– Randomly divide a group of 50 people with colds into two groups. Give each group a different medicine, and measure the length of the symptoms.

• This method is a comparative experiment.

Then explain which method would be most reliable

• Method A is least reliable, because there is no basis for comparison.

• Method B has a comparison group, but the members are self-selected, which could lead to bias.

• In method C, the members of each group are randomly selected, which makes the two groups theoretically similar except for the variable, the different medicines. This method is most reliable.

Homework 1-3

• Page 27 #1 – 33