A professor has a class of 40 students. Half of the students...

A professor has a class of 40 students. Half of the students chose to take a test after every chapter (chapter test condition) outside of class. The other half of the students chose to take in-class "unit tests." Unit tests covered four chapters. The professor finds no statistically significant differences between the groups on their scores on a comprehensive final exam. The professor then concludes that type of testing does not affect performance. a. Is this an experiment? b. Is the professor's conclusion reasonable? Why or why not?

A professor has a class of 40 students. Half of the students chose to take a test after every chapter (chapter test condition) outside of class. The other half of the students chose to take in-class "unit tests." Unit tests covered four chapters. The professor finds no statistically significant differences between the groups on their scores on a comprehensive final exam. The professor then concludes that type of testing does not affect performance.
a. Is this an experiment?
b. Is the professor's conclusion reasonable? Why or why not?

Key Concepts

Experimental Design

Experimental design involves planning a study in such a way that the researcher manipulates one or more variables, while controlling other factors, in order to establish cause and effect relationships. A true experiment typically requires random assignment to treatment and control groups to eliminate systematic biases, which differentiates it from other forms of study where such manipulation is absent.

Random Assignment

Random assignment is the process of assigning participants to different groups in an experiment solely by chance. This technique is essential because it ensures that any differences between and among the groups that might confound the results are minimized, thereby increasing the internal validity of the study.

Observational Study

An observational study is research where the investigators observe subjects and measure variables of interest without assigning treatments to the subjects. Because the participants are not randomly assigned, observational studies are more susceptible to various biases, and any association found cannot be confidently interpreted as a causal relationship.

Selection Bias

Selection bias occurs when participants self-select or are non-randomly assigned to groups, which may result in groups that differ in significant ways apart from the treatment. This bias can impact the validity of the study’s conclusions by confounding the effects of the treatment with preexisting differences between groups.

Causal Inference

Causal inference is the process of drawing conclusions about cause-and-effect relationships from data. It requires careful consideration of the study design, including the use of random assignment and control of confounding variables, to ensure that the observed effects can be attributed to the treatment rather than other factors.

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