Evidence-Based Medicine

Experimental study
A study in which the investigator studies the effect of intentionally altering one
or more factors under controlled conditions.
External validity (generalisabilty)
The validity of the results of a trial beyond that trial.
A randomised controlled trial (RCT) only provides direct evidence of causality
within that trial. It takes an additional logical step to apply this result more
generally. However, practically it is necessary to assume that results are
generalisable unless there is evidence to the contrary. If evidence is consistent
across different settings and in different populations (e.g. across ages and
countries) then there is evidence in favour of external validity. If there is only
evidence from atypical setting (e.g. teaching hospital when most cases are
seen in primary care) then one should be more sceptical about generalising the
results. Generalisability is not just a consequence of the entry requirements for
the trial, but also depends on the population from which the trial population was
drawn (see applicability).
F
Factorial design
A factorial design attempts to evaluate more than one intervention compared
with control in a single trial, by means of multiple randomisations.
False negative
A person with the target condition (defined by the gold standard) who has a
negative test result.
False positive
A person without the target condition (defined by the gold standard) who has a
positive test result.
Fixed effects
The “fixed effects” model of meta-analysis assumes, often unreasonably, that
the variability between the studies is exclusively because of a random sampling
variation around a fixed effect (see random effects below).
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