Quasi Random Assignment To Condition

Types of experiments

By definition, all experiments involve manipulation of one or more independent variables, and observing the effect on some outcome (dependent variable). Experiments can be done in the field or in a laboratory. They can involve human or animal subjects. What distinguishes the type of experiment is the degree to which the experimenter can assign subjects to conditions. Three types are described here: True, Quasi- and Single-subject experiments.

True experiments

In a true experiment, subjects are randomly assigned to the treatment conditions (levels of the independent variable). The only differences in the groups would be due to chance.

True experiments are excellent for showing a cause-and-effect relationship. Random assignment (or random assignment within matched groups) controls for extraneous variables.

They tend to be high on internal validity. It is clear what is being measured. There still might be bias in the overall research design, but at least variables associated with individuals are not a source of constant error (see sources of error in the Sampling module).


Quasi-experiments are sometimes called natural experiments because membership in the treatment level is determined by conditions beyond the control of the experimenter (subjects are already in the box). An experiment may seem to be a true experiment, but if the subjects have NOT been randomly assigned to the treatment condition, the experiment is a quasi- experiment (quasi = seeming, resembles).
Experiments that take advantage of natural occurrences are quasi-experiments, for example, comparing achievement level of first-born children with that of later-born children; or comparing student performance at two schools, one of which has a lower student-teacher ratio. The experimenter is unable to assign subjects to treatment level - the subjects are already in pre-existing groups.

One type of quasi experiment is to compare treatment versus control conditions, where the assignment has occurred as a result of some natural event. For example, comparing tranquilizer sales in a community struck by a hurricane, with sales in a similar community that was not affected by the hurricane. Or to compare drinking levels at two colleges with similar student bodies, one with an orientation programs with respect to alcohol use, and the other having no such program. More....

Another type of quasi-experiment is to compare pre- versus post- events or behavior, for example


Highway fatalities before and after increasing the speed limit
Gun sales in a community before and after a sensationalized killing
Number of migratory cranes in the Sacramento Valley before and after wetland habitat restoration
Activism by college students before and after an awareness campaign

Single-subject experiments

Instead of comparing behavior or performance of groups of people at a single point in time, a single-subject experiment involves a single case studied over a longer period of time. One individual or situation is exposed to the varying levels of the independent variable.

constant error - error resulting from bias; or from improper sampling or bad design

internal validity - degree to which a procedure measures what it is supposed to measure.

reliability - consistency in measurement; the repeatability or replicability of findings, stability of measurement over time.

The most simple single-subject research design is termed ABA, where A is the baseline (non-treatment or control) condition or phase. B refers to the introduction of the treatment factor. Behavior is recorded in both stages. Then there is a return to A to see if in fact it was B that brought about the change. An example might be treating a hyperactive child with a drug. Stage A involves recording the child's behavior before any treatment, e.g., how many disruptive events in the classroom within a specified period of time. Stage B would involve the same measurement after the child has been treated. If B (the treatment) makes a difference, returning to level A (no treatment) should result in a return of the disruptive behavior. The basic research design can include a second treatment phase -- ABAB, thereby increasing the reliability and internal validity of the results.

The subject of a single-subject experiment might be an entire community. For example, police in a small city introduced helicopter patrols to see whether or not they resulted in a decrease in burglaries. Here is a figure showing the results.

This is an ABABA design.

The occurrence of burglaries tended to be higher when the helicopter was not on patrol. In setting policy the officials would have to weigh the benefit against the added cost.

true vs. quasi-andlab vs. field studies

It is easy to confuse setting with experiment type. Use the table below to keep them straight.

LOCATIONTrueQuasi- (natural)
less common common

Although quasi-experiments are not commonly done in a lab, some of the subject variables in lab-run experiments, such as gender or socioeconomic level, are not subject to random assignment.

Take Self-test #3

Next section: Strengths and limitations

Experiments and Quasi-Experiments

An experiment is a study in which the researcher manipulates the level of some independent variable and then measures the outcome. Experiments are powerful techniques for evaluating cause-and-effect relationships. Many researchers consider experiments the "gold standard" against which all other research designs should be judged. Experiments are conducted both in the laboratory and in real life situations.

Types of Experimental Design

There are two basic types of research design:

  • True experiments
  • Quasi-experiments

The purpose of both is to examine the cause of certain phenomena.

True experiments, in which all the important factors that might affect the phenomena of interest are completely controlled, are the preferred design. Often, however, it is not possible or practical to control all the key factors, so it becomes necessary to implement a quasi-experimental research design.

Similarities between true and quasi-experiments:

  • Study participants are subjected to some type of treatment or condition
  • Some outcome of interest is measured
  • The researchers test whether differences in this outcome are related to the treatment

Differences between true experiments and quasi-experiments:

  • In a true experiment, participants are randomly assigned to either the treatment or the control group, whereas they are not assigned randomly in a quasi-experiment
  • In a quasi-experiment, the control and treatment groups differ not only in terms of the experimental treatment they receive, but also in other, often unknown or unknowable, ways. Thus, the researcher must try to statistically control for as many of these differences as possible
  • Because control is lacking in quasi-experiments, there may be several "rival hypotheses" competing with the experimental manipulation as explanations for observed results

Key Components of Experimental Research Design

The Manipulation of Predictor Variables

In an experiment, the researcher manipulates the factor that is hypothesized to affect the outcome of interest. The factor that is being manipulated is typically referred to as the treatment or intervention. The researcher may manipulate whether research subjects receive a treatment (e.g., antidepressant medicine: yes or no) and the level of treatment (e.g., 50 mg, 75 mg, 100 mg, and 125 mg).

Suppose, for example, a group of researchers was interested in the causes of maternal employment. They might hypothesize that the provision of government-subsidized child care would promote such employment. They could then design an experiment in which some subjects would be provided the option of government-funded child care subsidies and others would not. The researchers might also manipulate the value of the child care subsidies in order to determine if higher subsidy values might result in different levels of maternal employment.

Random Assignment

  • Study participants are randomly assigned to different treatment groups
  • All participants have the same chance of being in a given condition
  • Participants are assigned to either the group that receives the treatment, known as the "experimental group" or "treatment group," or to the group which does not receive the treatment, referred to as the "control group"
  • Random assignment neutralizes factors other than the independent and dependent variables, making it possible to directly infer cause and effect

Random Sampling

Traditionally, experimental researchers have used convenience sampling to select study participants. However, as research methods have become more rigorous, and the problems with generalizing from a convenience sample to the larger population have become more apparent, experimental researchers are increasingly turning to random sampling. In experimental policy research studies, participants are often randomly selected from program administrative databases and randomly assigned to the control or treatment groups.

Validity of Results

The two types of validity of experiments are internal and external. It is often difficult to achieve both in social science research experiments.

Internal Validity

  • When an experiment is internally valid, we are certain that the independent variable (e.g., child care subsidies) caused the outcome of the study (e.g., maternal employment)
  • When subjects are randomly assigned to treatment or control groups, we can assume that the independent variable caused the observed outcomes because the two groups should not have differed from one another at the start of the experiment
  • For example, take the child care subsidy example above. Since research subjects were randomly assigned to the treatment (child care subsidies available) and control (no child care subsidies available) groups, the two groups should not have differed at the outset of the study. If, after the intervention, mothers in the treatment group were more likely to be working, we can assume that the availability of child care subsidies promoted maternal employment

One potential threat to internal validity in experiments occurs when participants either drop out of the study or refuse to participate in the study. If particular types of individuals drop out or refuse to participate more often than individuals with other characteristics, this is called differential attrition. For example, suppose an experiment was conducted to assess the effects of a new reading curriculum. If the new curriculum was so tough that many of the slowest readers dropped out of school, the school with the new curriculum would experience an increase in the average reading scores. The reason they experienced an increase in reading scores, however, is because the worst readers left the school, not because the new curriculum improved students' reading skills.

External Validity

  • External validity is also of particular concern in social science experiments
  • It can be very difficult to generalize experimental results to groups that were not included in the study
  • Studies that randomly select participants from the most diverse and representative populations are more likely to have external validity
  • The use of random sampling techniques makes it easier to generalize the results of studies to other groups

For example, a research study shows that a new curriculum improved reading comprehension of third-grade children in Iowa. To assess the study's external validity, you would ask whether this new curriculum would also be effective with third graders in New York or with children in other elementary grades.

Glossary terms related to validity:


It is particularly important in experimental research to follow ethical guidelines. Protecting the health and safety of research subjects is imperative. In order to assure subject safety, all researchers should have their project reviewed by the Institutional Review Boards (IRBS). The National Institutes of Health supplies strict guidelines for project approval. Many of these guidelines are based on the Belmont Report (pdf).

The basic ethical principles:

  • Respect for persons -- requires that research subjects are not coerced into participating in a study and requires the protection of research subjects who have diminished autonomy
  • Beneficence -- requires that experiments do not harm research subjects, and that researchers minimize the risks for subjects while maximizing the benefits for them
  • Justice -- requires that all forms of differential treatment among research subjects be justified

Advantages and Disadvantages of Experimental Design


The environment in which the research takes place can often be carefully controlled. Consequently, it is easier to estimate the true effect of the variable of interest on the outcome of interest.


It is often difficult to assure the external validity of the experiment, due to the frequently nonrandom selection processes and the artificial nature of the experimental context.


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