1.1.3 - Types of Studies

Now that we know how to collect data, the next step is to determine the type of study. The type of study will determine what type of relationship we can conclude.

There are predominantly two different types of studies:

A study where a researcher records or observes the observations or measurements without manipulating any variables. These studies show that there may be a relationship but not necessarily a cause and effect relationship.
A study that involves some random assignment* of a treatment; researchers can draw cause and effect (or causal) conclusions. An experimental study may also be called a scientific study or an experiment.

Note! Random selection (a probability method of sampling) is not random assignment (as in an experiment). In an ideal world you would have a completely randomized experiment; one that incorporates random sampling and random assignment.

Example 1-2: Types of Studies Section

Quiz and Exam Score Studies

Let's say that there is an option to take quizzes throughout this class. In an observational study, we may find that better students tend to take the quizzes and do better on exams. Consequently, we might conclude that there may be a relationship between quizzes and exam scores.

In an experimental study, we would randomly assign quizzes to specific students to look for improvements. In other words, we would look to see whether taking quizzes causes higher exam scores.

Causation Section

It is very important to distinguish between observational and experimental studies since one has to be very skeptical about drawing cause and effect conclusions using observational studies. The use of random assignment of treatments (i.e. what distinguishes an experimental study from an observational study) allows one to employ cause and effect conclusions.

Ethics is an important aspect of experimental design to keep in mind. For example, the original relationship between smoking and lung cancer was based on an observational study and not an assignment of smoking behavior.

Try It! Section

We want to decide whether Advil or Tylenol is more effective in reducing fever.

Method 1

Ask the subjects which one they use and ask them to rate the effectiveness. Is this an observational study or experimental study?
This is an observational study since we just observe the data and have no control on which subject to use what type of treatment.

Method 2

Randomly assign half of the subjects to take Tylenol and the other half to take Advil. Ask the subjects to rate the effectiveness. Is this an observational study or experimental study?
This is an experimental study since we can decide which subject to use what type of treatment. Thus the self selection bias will be eliminated.