# Systematic Review Terminology

**Attrition:**
subject units lost during the
experimental/investigational period than cannot be included in the analysis
(e.g. units removed due to deleterious side-effects caused by the
intervention).

**
Bias (synonym: systematic
error):** the
distortion of the
outcome, as a result of a
known or unknown variable other than
intervention
(i.e. the tendency to produce results that depart from the “true” result).

**
Confounding variable
(synonym: co-variate):**
a variable associated with the
outcome, which
distorts the effect of
intervention.

**
Effectiveness:**
the extent to which an
intervention produces a
beneficial
outcome under ordinary
circumstances (i.e. does the
intervention
work?).

**
Effect size:**
the observed association between the
intervention
and outcome, where the improvement/decrement
of the
outcome is described in deviations from
the mean.

**
Efficacy:**
the extent to which an
intervention produces a
beneficial
outcome under ideally controlled
circumstances (i.e. can the
intervention
work?).

**
Efficiency: **
the extent to which the
effect of the
intervention on the
outcome
represents value for money (i.e. the
balance between cost and
outcome).

**
Evidence-based health care:**
extends the application of the principles of evidence-based medicine to all
professions associated with health care, including purchasing and management.

**
Evidence-based
medicine (EBM):**
is the conscientious, explicit and judicious use of current best evidence in
making decisions about the care of individual patients. The practice of
evidence-based medicine means integrating individual clinical expertise with the
best available external clinical evidence from systematic research.

**
Fixed effects model:
**a mathematical model
that combines the results of studies that assume the effect of the intervention
is constant in all
subject populations
studied. Only within-study variation is included when assessing the uncertainty
of results (in contrast to a
random effects model).

**
Forest plot: **
a plot illustrating
individual
effect sizes observed in studies
included within a
systematic review
(incorporating the
summary effect if
meta-analysis is used).

**
Funnel plot:**
a graphical method of assessing bias; the
effect size
of each study is plotted against some measure of study information (e.g. sample
size; if the shape of the plot resembles an inverted funnel, it can be stated
that there is no evidence of publication bias within the
systematic review).

**
Heterogeneity: **
the variability between
studies in terms of key characteristics (i.e. ecological variables) quality
(i.e. methodology) or effect (i.e. results). Statistical tests**
**of heterogeneity
may be used to assess whether the observed variability in
effect size (i.e. study results) is greater
than that expected to occur purely by chance.

**
Intervention: **
the policy or management
action under scrutiny within the
systematic review.

**
Mean difference:**
the difference between the means of two groups of measurements.

**
Meta-analysis:**
a quantitative method employing statistical techniques, to combine and summarise
the results of studies that address the same question.

**
Meta-regression:**
A multivariable model investigating
effect size
from individual studies, generally weighted by sample size, as a function of
various study characteristics (i.e. to investigate whether study characteristics
are influencing
effect size).

**Outcome:
**the effect of the
intervention in a form that can be reliably
measured.

**
Power: **
the ability to demonstrate
an association where one exists (i.e. the larger the sample size, the greater
the power and the lower the probability of the association remaining
undetected).

**
Precision: **
the proportion of relevant
articles identified by a search strategy as a percent of all articles** **
found (i.e. a measure of the ability of a search strategy to exclude irrelevant
articles).**
**

**Protocol:**the set of steps to be followed in a systematic review. It describes the rationale for the review, the objective(s), and the methods that will be used to locate, select and critically appraise studies, and to collect and analyse data from the included studies.

**
Publication bias:**
the possible result of an unsystematic approach to a
review (e.g. research that generates a negative result is less likely to
be published than that with a positive result, and this may therefore give a
misleading assessment of the impact of an
intervention). Publication bias can be examined via a
funnel plot.

**
Random effects model:
**a mathematical model for
combining the results of studies that allow for variation in the effect of the
intervention amongst the
subject populations studied. Both within-study
variation and between-study variation is included when assessing the uncertainty
of results (in contrast to a
fixed effects model).

**
Review:**
an article that summarises a number of primary studies and discusses the
effectiveness of a particular
intervention. It may or may not be a
systematic review.

**
Search strategy: **
an *a priori*
description of the methodology, to be used to locate and identify research
articles pertinent to a
systematic review, as
specified within the relevant
protocol. It
includes a list of search terms, based on the
subject,
intervention and
outcome of the review, to be used when searching electronic databases,
websites, reference lists and when engaging with personal contacts. If required,
the strategy may be modified once the search has commenced.

**
Sensitivity:**
the proportion of relevant articles identified by a search strategy as a
percentage of all relevant articles on a given topic (i.e. the degree of
comprehensiveness of the search strategy and its ability to identify all
relevant articles on a
subject).

**
Sensitivity analysis:
**repetition of the
analysis using different sets of assumptions (with regard to the methodology or
data) in order to determine the impact of variation arising from these
assumptions, or uncertain decisions, on the results of a
systematic review.

**
Standardised mean difference
(SMD): **an
effect size measure used when studies have
measured the same outcome using different
scales. The mean difference is divided by an estimate of the within-group
variance to produce a standardised value without units.

**
Study quality:**
the degree to which a study seeks to minimise
bias.

**
Subgroup analysis:
**used to determine if the
effects of an
intervention vary between
subgroups in the
systematic review. Subgroups
may be pre-defined according to differences in
subject populations,
intervention,
outcome and study design.

**
Subject: **
the unit of study to which
the intervention is to be applied.

**
Summary effect size:
**the pooled
effect size, generated by combining individual
effect sizes in a
meta-analysis.

**
Systematic review (synonym:
systematic overview): **
a review of a clearly
formulated question that uses systematic and explicit methods to identify,
select and critically appraise relevant research, and to collect and analyse
data from the studies that are included within the review. Statistical methods (meta-analysis)
may or may not be used to analyse and summarise the results of the included
studies.

**
Weighted mean difference (WMD):**a
summary effect size measure for continuous
data where studies that have measured the
outcome
on the same scale have been pooled.

This glossary has been compiled and adapted from:

Khan, K.S., Kunz, R., Kleijnen, J., and Antes, G. (2003). Systematic Reviews to Support Evidence-Based Medicine: how to apply findings of healthcare research. Royal Society of Medical Press Ltd. London, UK.

NHS Centre for Reviews and Dissemination. (2001). Undertaking Systematic Review of Research on Effectiveness, ed. K.S. Khan, G. ter Riet, J. Glanville, A.J. Sowden, and J. Kleijnen. NHS CRD Report No. 4. University of York, York, UK.