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
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