What to Know About 2D: Observational Studies

learning GOALS

At the end of this page, you should feel comfortable performing these skills:

  • Explain why observational studies are used.
  • Explain what a confounding variable is in an observational study.
  • Explain the difference between an experimental study and an observational study.

In the next activity, you will learn about observational studies and learn to distinguish between experiments and observational studies.

You previously learned about experiments, or experimental studies. An experimental study is a statistical study based on data collected from designed experiments and is useful for determining cause and effect. In such a study, a treatment is imposed by the researcher to allow for a comparison between groups. In this What to Know page, you’ll learn how to distinguish between experimental studies and observational studies.

Observational Studies

In an observational study, a researcher will observe an outcome without changing who is and who is not exposed to some sort of treatment. Instead of trying to assess cause and effect, an observational study is typically used when the goal is to learn about characteristics of a population or to compare groups within a population with respect to some characteristic. Sometimes observational studies are preferred over experimental studies because they might be the only way certain types of questions can be studied. Observational studies may allow a researcher to avoid assigning participants of a study to a treatment that would be considered unethical. However, due to lack of control and assignment to a treatment, observational studies may inherently be at risk of containing confounding biases, which are outside of the control of the researcher. A confounding variable is a variable that was not accounted for in a study and may actually influence other variables in a study.

Video Placement

[Perspective Video: a 3-instructor video that previews all the ideas on this page — experiment vs observational study, confounding variables, and why observational studies are necessary (i.e., why experiments would sometimes be unethical to conduct.]

Now you try to answer some questions about an observational study. For Questions 1–4, consider the following scenario.

A researcher wants to explore how smoking habits relate to depression. She has a group of self-described smokers take a survey and assign a numerical value to describe their level of depression. The survey also asks participants to record their heights, weights, and ages. Similarly, she has a group of self-described non-smokers take the same survey.

question 1

1) In this study, level of depression, height, weight, and age are all ____________.

  1. a) populations
  2. b) samples
  3. c) variables

question 2

2) This is an observational study because _____________.

  1. a) the researcher assigns each participant to the smoking or non-smoking group
  2. b) the researcher collects information from the participants without assigning a treatment

question 3

3) Determine whether this statement is true or false: When studying the relationship between smoking habits and depression, an observational study is best since it would be unethical to force participants to begin smoking.

question 4

4) Determine whether this statement is true or false: Based on the responses, the researcher notes that several of the smokers also reported higher weights than the non-smokers. Weight may be a confounding variable in this study.

Observational Study vs. Experiment

Video Placement

[Worked Example/Perspective: a 3-instructor video that gives pointers on how to distinguish between an experiment and an observational study in the style of Questions 5 and 6]

Now you try identifying which type of study was used given a brief description. For Questions 5 and 6, select the most correct statement for the given scenario.

question 5

5) A geography professor divides his classroom into in-state students and out-of-state students and quizzes them on local landmarks.

  1. a) This is an experimental study since the professor has divided his class into two groups.
  2. b) This is an experimental study since the professor has not assigned a treatment.
  3. c) This is an observational study since the professor has divided his class into two groups.
  4. d) This is an observational study since the professor has not assigned a treatment.

question 6

6) A pharmaceutical company develops a new blood pressure medication and recruits 300 participants for a study. For this study, 100 people are randomly assigned to the new medication, 100 are randomly assigned to the old medication, and 100 are given a placebo (no medication). Blood pressure levels of the participants are monitored over time.

  1. a) This is an experimental study since the company randomly assigns participants to groups and provides a treatment to each group.
  2. b) This is an experimental study since the blood pressure levels of the participants are monitored over time.
  3. c) This is an observational study since the company randomly assigns participants to groups and provides a treatment to each group.
  4. d) This is an observational study since the blood pressure levels of the participants are monitored over time.

Looking ahead

In the next class, you will be discussing characteristics of a very recent study from the Centers of Disease Control and Prevention. In this study, researchers followed 3,950 health care personnel, first responders, and other essential frontline workers for 13 consecutive weeks to observe how effective Messenger RNA COVID-19 vaccines have been in a real-world setting. The study can be found here: https://www.cdc.gov/mmwr/volumes/70/wr/mm7013e3.htm.

question 7

Answer the following questions about the study.

 

Part A: Who is being studied?

 

Part B: What is being observed for each participant in the study?

Summary

In this What to Know page, you …  Let’s summarize these new skills.

  • In Question 1, you practiced vocabulary you saw previously.
  • In Question 2, you identified an observational study.
  • In Question 3, you stated why observational studies are used.
  • In Question 4, you identified a confounding variable in an observational study.
  • In Question 5 – 6, identified the difference between an experimental study and an observational study.
  • In Question 7, you prepared for the upcoming activity. Don’t forget to have your answer ready when you begin the activity!

If you feel comfortable with these ideas, it’s time to move on to Forming Connections.