Design used in experimental research in which groups are randomly formed and that controls most sources of invalidity is known as

Question

Prepp Team · Accepted Answer

Understanding Experimental Research Designs

Experimental research is a type of study where the researcher manipulates one or more variables (independent variables) to observe their effect on another variable (dependent variable), while controlling for other factors. A key aspect of experimental research is the design used, which determines the rigor and validity of the findings.

True-Experimental Design Explained

The question describes a research design characterized by:

Groups are randomly formed.
It controls most sources of invalidity.

This description perfectly matches the definition of a true-experimental design. In a true experiment, participants are randomly assigned to different groups, typically a control group and one or more experimental groups. Random assignment is crucial because it helps ensure that the groups are comparable at the start of the experiment, minimizing pre-existing differences that could influence the outcome. This strong control over potential confounding variables allows researchers to make more confident claims about cause and effect.

Why Other Designs Don't Fully Fit

Let's briefly look at the other options to see why they don't meet the criteria described in the question:

Pre-experimental design: These designs are the simplest form of experimental research. They often lack random assignment and/or a control group. Because they have limited control over extraneous variables, they are susceptible to many sources of invalidity, making it difficult to establish causality. Examples include the one-shot case study or the one-group pretest-posttest design.
Quasi-experimental design: These designs involve manipulating an independent variable but lack random assignment of participants to groups. Instead, they often use pre-existing groups (e.g., students in different classrooms). While they attempt to control for some variables, the absence of random assignment means there might be inherent differences between groups that could affect the results, increasing the risk of invalidity compared to true experiments.
Ex-post-facto design: This is not an experimental design in the traditional sense. In ex-post-facto research (also known as causal-comparative research), the researcher studies events or conditions that have already occurred. The independent variable is not manipulated; instead, the researcher compares groups that already differ on some characteristic. This design cannot establish cause and effect conclusively because there is no manipulation or random assignment, making it highly vulnerable to confounding variables.

Based on the definition provided in the question—random group formation and control over invalidity—the true-experimental design is the most appropriate answer.

Comparison of Research Designs
Design Type	Random Assignment	Control Group	Control over Invalidity
Pre-experimental	No	Often No	Low
True-experimental	Yes	Yes	High
Quasi-experimental	No	Often Yes	Moderate
Ex-post-facto	No	Often No	Low (non-experimental)

Conclusion

The design used in experimental research where groups are randomly formed and that controls most sources of invalidity is known as the true-experimental design. This design provides the strongest evidence for cause-and-effect relationships.

Revision Table: Key Research Designs

Term	Definition	Key Feature(s)
True-experimental Design	A research design using random assignment to groups to test causality.	Random assignment, manipulation, control group, high internal validity.
Pre-experimental Design	Simple designs lacking random assignment or control groups, weak in causality.	No random assignment, limited control.
Quasi-experimental Design	Designs lacking random assignment but manipulating variables, often using existing groups.	No random assignment, manipulation, uses existing groups.
Ex-post-facto Design	Non-experimental design studying pre-existing conditions or differences.	No manipulation, no random assignment, retrospective.

Additional Information: Validity in Experimental Research

Controlling sources of invalidity is a primary goal of true-experimental designs. There are two main types of validity:

Internal Validity: This refers to the extent to which a researcher can confidently conclude that the independent variable caused the observed effect on the dependent variable. True experiments with random assignment maximize internal validity by controlling for confounding variables. Threats to internal validity include history, maturation, testing, instrumentation, selection bias, mortality, and regression to the mean.
External Validity: This refers to the extent to which the findings of a study can be generalized to other populations, settings, and times. While true experiments enhance internal validity, the controlled environment might sometimes make it harder to generalize findings to real-world situations. Researchers often balance the need for high internal validity with the desire for good external validity.

The strength of the true-experimental design lies in its ability to address threats to internal validity effectively, primarily through random assignment and controlled manipulation.