Repeated measures design
Encyclopedia
The repeated measures design uses the same subjects with every condition of the research, including the control. For instance, repeated measures are collected in a longitudinal study
in which change over time is assessed. Other studies compare the same measure under two or more different conditions. For instance, to test the effects of caffeine on cognitive function, a subject's math ability might be tested once after they consume caffeine and another time when they consume a placebo.
. A crossover study is a longitudinal study
in which subjects receive a sequence
of different treatments (or exposures). While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article. Crossover designs are common for experiments in many scientific disciplines, for example psychology, education
, pharmaceutical science, and health-care, especially medicine.
Randomized
, controlled, crossover experiments are especially important in health-care. In a randomized clinical trial
, the subjects are randomly assigned
treatments. When the randomized
clinical trial
is a repeated measures design, the subjects are randomly assigned
to a sequence
of treatments. A crossover clinical trial
is a repeated-measures design in which each patient is randomly assigned to a sequence
of treatments, including at least two treatments (of which one "treatment" may be a standard treatment
or a placebo
): Thus each patient crosses over from one treatment to another.
Nearly all crossover designs have "balance", which means that all subjects should receive the same number of treaments and that all subjects participate for the same number of periods. In most crossover trials, each subject receives all treatments.
However, many repeated-measures designs are not crossover studies: The longitudinal study of the sequential effects of repeated treatments need not use any "crossover
", for example (Vonesh & Chinchilli; Jones & Kenward).
Both types, however, have the goal of controlling for practice effects.
There are also several threats to the internal validity of this design, namely a regression threat (when subjects are tested several times, their scores tend to regress towards the mean), a maturation threat (subjects may change during the course of the experiment) and an history threat (events outside the experiment that may change the response of subjects between the repeated measures).
Longitudinal study
A longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
in which change over time is assessed. Other studies compare the same measure under two or more different conditions. For instance, to test the effects of caffeine on cognitive function, a subject's math ability might be tested once after they consume caffeine and another time when they consume a placebo.
Crossover studies, an example of a repeated measures design
A popular repeated-measures design is the crossover studyCrossover study
A crossover study is a longitudinal study in which subjects receive a sequence of different treatments . While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article...
. A crossover study is a longitudinal study
Longitudinal study
A longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
in which subjects receive a sequence
Tuple
In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...
of different treatments (or exposures). While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article. Crossover designs are common for experiments in many scientific disciplines, for example psychology, education
Education
Education in its broadest, general sense is the means through which the aims and habits of a group of people lives on from one generation to the next. Generally, it occurs through any experience that has a formative effect on the way one thinks, feels, or acts...
, pharmaceutical science, and health-care, especially medicine.
Randomized
Randomization
Randomization is the process of making something random; this means:* Generating a random permutation of a sequence .* Selecting a random sample of a population ....
, controlled, crossover experiments are especially important in health-care. In a randomized clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
, the subjects are randomly assigned
Randomization
Randomization is the process of making something random; this means:* Generating a random permutation of a sequence .* Selecting a random sample of a population ....
treatments. When the randomized
Randomization
Randomization is the process of making something random; this means:* Generating a random permutation of a sequence .* Selecting a random sample of a population ....
clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
is a repeated measures design, the subjects are randomly assigned
Randomization
Randomization is the process of making something random; this means:* Generating a random permutation of a sequence .* Selecting a random sample of a population ....
to a sequence
Tuple
In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...
of treatments. A crossover clinical trial
Clinical trial
Clinical trials are a set of procedures in medical research and drug development that are conducted to allow safety and efficacy data to be collected for health interventions...
is a repeated-measures design in which each patient is randomly assigned to a sequence
Tuple
In mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...
of treatments, including at least two treatments (of which one "treatment" may be a standard treatment
Standard treatment
Standard treatment . The treatment that is normally provided to people with a given condition. In many studies, a control group receives the standard treatment while a treatment group receives the experimental treatment...
or a placebo
Placebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...
): Thus each patient crosses over from one treatment to another.
Nearly all crossover designs have "balance", which means that all subjects should receive the same number of treaments and that all subjects participate for the same number of periods. In most crossover trials, each subject receives all treatments.
However, many repeated-measures designs are not crossover studies: The longitudinal study of the sequential effects of repeated treatments need not use any "crossover
Crossover study
A crossover study is a longitudinal study in which subjects receive a sequence of different treatments . While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article...
", for example (Vonesh & Chinchilli; Jones & Kenward).
Uses of a repeated measures design
- Conduct an experiment when few participants are available: The repeated measure design reduces the variance of estimates of treatment-effects, allowing statistical inference to be made with fewer subjects.
- Conduct experiment more efficiently: Repeated measures designs allow many experiments to be completed more quickly, as only a few groups need to be trained to complete an entire experiment. For example, there are many experiments where each condition takes only a few minutes, whereas the training to complete the tasks take as much, if not more time.
- Study changes in participants’ behavior over time: Repeated measures designs allow researchers to monitor how the participants change over the passage of time, both in the case of long-term situations like longitudinal studiesLongitudinal studyA longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
and in the much shorter-term case of practice effects.
Practice effects
Practice effects occur when a participant in an experiment is able to perform a task and then perform it again at some later time. Generally, they either have a positive (subjects become better at performing the task) or negative (subjects become worse at performing the task) effect. Repeated measures designs are almost always affected by practice effects; the primary exception to this rule is in the case of a longitudinal study. How well these are measured is controlled by the exact type of repeated measure design that is used.Both types, however, have the goal of controlling for practice effects.
Advantages
The primary strengths of the repeated measures design is that it makes an experiment more efficient and helps keep the variability low. This helps to keep the validity of the results higher, while still allowing for smaller than usual subject groups.Disadvantages
A disadvantage to the repeated measure design is that it may not be possible for each participant to be in all conditions of the experiment (i.e. time constraints, location of experiment, etc.).There are also several threats to the internal validity of this design, namely a regression threat (when subjects are tested several times, their scores tend to regress towards the mean), a maturation threat (subjects may change during the course of the experiment) and an history threat (events outside the experiment that may change the response of subjects between the repeated measures).
Exploration of longitudinal data
(Comprehensive treatment of theory and practice)- Conaway, M. (1999, October 11). Repeated Measures Design. Retrieved February 18, 2008, from http://biostat.mc.vanderbilt.edu/twiki/pub/Main/ClinStat/repmeas.PDF
- Minke, A. (1997, January). Conducting Repeated Measures Analyses: Experimental Design Considerations. Retrieved February 18, 2008, from Ericae.net: http://ericae.net/ft/tamu/Rm.htm
- Shaughnessy, J. J. (2006). Research Methods in Psychology. New York: McGraw-Hill.
See also
- Analysis of varianceAnalysis of varianceIn statistics, analysis of variance is a collection of statistical models, and their associated procedures, in which the observed variance in a particular variable is partitioned into components attributable to different sources of variation...
- Clinical trial protocolClinical trial protocolA clinical trial protocol is a document that describes the objective, design, methodology, statistical considerations, and organization of a clinical trial...
- Crossover studyCrossover studyA crossover study is a longitudinal study in which subjects receive a sequence of different treatments . While crossover studies can be observational studies, many important crossover studies are controlled experiments, which are discussed in this article...
- Design of experimentsDesign of experimentsIn general usage, design of experiments or experimental design is the design of any information-gathering exercises where variation is present, whether under the full control of the experimenter or not. However, in statistics, these terms are usually used for controlled experiments...
- Glossary of experimental designGlossary of experimental design- Glossary :* Alias: When the estimate of an effect also includes the influence of one or more other effects the effects are said to be aliased . For example, if the estimate of effect D in a four factor experiment actually estimates , then the main effect D is aliased with the 3-way interaction ABC...
- Longitudinal studyLongitudinal studyA longitudinal study is a correlational research study that involves repeated observations of the same variables over long periods of time — often many decades. It is a type of observational study. Longitudinal studies are often used in psychology to study developmental trends across the...
- Growth curveGrowth curveA growth curve is an empirical model of the evolution of a quantity over time. Growth curves are widely used in biology for quantities such as population size, body height or biomass...
- Missing data
- Multivariate analysisMultivariate analysisMultivariate analysis is based on the statistical principle of multivariate statistics, which involves observation and analysis of more than one statistical variable at a time...
- Observational studyObservational studyIn epidemiology and statistics, an observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator...
- Optimal designOptimal designOptimal designs are a class of experimental designs that are optimal with respect to some statistical criterion.In the design of experiments for estimating statistical models, optimal designs allow parameters to be estimated without bias and with minimum-variance...
- Panel analysisPanel analysisPanel analysis is statistical method, widely used in social science, epidemiology, and econometrics, which deals with two-dimensional panel data. The data are usually collected over time and over the same individuals and then a regression is run over these two dimensions...
- Panel dataPanel dataIn statistics and econometrics, the term panel data refers to multi-dimensional data. Panel data contains observations on multiple phenomena observed over multiple time periods for the same firms or individuals....
- Panel study
- RandomizationRandomizationRandomization is the process of making something random; this means:* Generating a random permutation of a sequence .* Selecting a random sample of a population ....
- Randomized controlled trialRandomized controlled trialA randomized controlled trial is a type of scientific experiment - a form of clinical trial - most commonly used in testing the safety and efficacy or effectiveness of healthcare services or health technologies A randomized controlled trial (RCT) is a type of scientific experiment - a form of...
- Repeated measures design
- SequenceTupleIn mathematics and computer science, a tuple is an ordered list of elements. In set theory, an n-tuple is a sequence of n elements, where n is a positive integer. There is also one 0-tuple, an empty sequence. An n-tuple is defined inductively using the construction of an ordered pair...
- Statistical inferenceStatistical inferenceIn statistics, statistical inference is the process of drawing conclusions from data that are subject to random variation, for example, observational errors or sampling variation...
- Treatment effectEffect sizeIn statistics, an effect size is a measure of the strength of the relationship between two variables in a statistical population, or a sample-based estimate of that quantity...