Study Design 101: Glossary

Blinding/Masking

When the groups that have been randomly selected from a population do not know whether they are in the control group or the experimental group.

Case

A patient with the disease or outcome of interest.

Case Series

A report about a small group of similar cases.

Causation

Being able to show that an independent variable directly causes the dependent variable. This is generally very difficult to demonstrate in most study designs.

Cochrane Database of Systematic Reviews

A highly-regarded database of systematic reviews prepared by The Cochrane Collaboration, an international group of individuals and institutions who review and analyze the published literature.

Cohort (population)

A group that shares the same characteristics among its members.

Confounding

When an exposure and an outcome are both strongly associated with a third variable.

Confounding Variables

Variables that cause/prevent an outcome from occurring outside of or along with the variable being studied. These variables render it difficult or impossible to distinguish the relationship between the variable and outcome being studied.

Control

A patient who does not have the disease or outcome.

Correlation

A relationship between two variables, but not necessarily a causation relationship.

Double Blinding/Masking

When the researchers conducting a blinded study do not know which participants are in the control group of the experimental group.

Exclusion Criteria

The set of conditions that characterize some individuals which result in being excluded in the study (i.e. other health conditions, taking specific medications, etc.). Since systematic reviews seek to include all relevant studies, exclusion criteria are not generally utilized in this situation.

Inclusion Criteria

The set of conditions that studies must meet to be included in the review (or for individual studies—the set of conditions that participants must meet to be included in the study; often comprises age, gender, disease type and status, etc.).

Matched Design

Each case is matched individually with a control according to certain characteristics such as age and gender. It is important to remember that the concordant pairs (pairs in which the case and control are either both exposed or both not exposed) tell us nothing about the risk of exposure separately for cases or controls..

Null Hypothesis

That the relationship between the independent and dependent variables the researchers believe they will prove through conducting a study does not exist. To "reject the null hypothesis" is to say that there is a relationship between the variables.

Observed Assignment

The method of assignment of individuals to study and control groups in observational studies when the investigator does not intervene to perform the assignment.

Population Bias/Volunteer Bias

A sample may be skewed by those who are selected or self-selected into a study. If only certain portions of a population are considered in the selection process, the results of a study may have poor validity.

Population/Cohort

A group that shares the same characteristics among its members.

Preplanned Case-Observations

A case in which symptoms are elicited to study disease mechanisms. (Ex. Having a patient sleep in a lab to do brain imaging for a sleep disorder).

Prospective Study

A study that moves forward in time, or that the outcomes are being observed as they occur, as opposed to a retrospective study, which looks back on outcomes that have already taken place.

Publication Bias

A phenomenon in which studies with positive results have a better chance of being published, are published earlier, and are published in journals with higher impact factors. Therefore, conclusions based exclusively on published studies can be misleading.

Randomization

Any of a number of mechanisms used to assign participants into different groups with the expectation that these groups will not differ in any significant way other than treatment and outcome.

Research (Alternative) Hypothesis

The relationship between the independent and dependent variables that researchers believe they will prove through conducting a study.

Sensitivity

The relationship between what is considered a symptom of an outcome and the outcome itself; or the percent chance of not getting a false positive (see formulas).

Specificity

The relationship between not having a symptom of an outcome and not having the outcome itself; or the percent chance of not getting a false negative (see formulas).

Type 1 Error

Rejecting a null hypothesis when it is in fact true. This is also known as an error of commission.

Type 2 Error

The failure to reject a null hypothesis when it is in fact false. This is also known as an error of omission.

Unmatched Design

The controls are a sample from a suitable non-affected population.