GLOSSARY OF TERMS

M

Masking
See blinding.

Mean (synonyms: arithmetic mean, average)
The average value calculated by adding all the observations and dividing by the number of  observations.

MEDLINE (MEDlars onLINE)
An electronic database produced by the United States National Library of Medicine. It  indexes millions of articles in selected (about 3,700) journals. It is available through most medical libraries, and can be accessed on CD-ROM, the Internet and by other means.  Years of coverage: 1966 to present.

MeSH headings (Medical Subject  Headings)
Terms used by the United States National Library of Medicine to index articles in Index  Medicus and MEDLINE. Designed to reduce problems that arise from,  for example, differences in British and American spelling. The MeSH system has a tree  structure in which broad subject terms branch into a series of progressively narrower subject terms.

Meta-analysis
The use of statistical techniques in a systematic review to integrate the results of the included studies. Also used to refer to systematic reviews that use meta-analysis.

Meta-regression
Multivariate meta-analytic techniques, such as logistic regression, used to explore the  relationship between study characteristics (e.g. allocation concealment, baseline risk, timing of the intervention) and study results (the magnitude of effect observed in each study) in a systematic review.

Methodological quality (synonyms: validity, internal validity)
The extent to which the design and conduct of a trial are likely to have prevented systematic errors (bias). Variation in quality can explain variation  in the results of trials included in a systematic review. More rigorously designed (better 'quality') trials are more likely to yield results that are closer to the 'truth'. See  also external validity, validity.

Minimization
A method of allocation used, particularly in small trials, to provide comparison groups that are closely similar for several variables. It can be done with or without a component of randomization. It is best performed centrally with the aid of a computer program to ensure concealment of allocation.

Multiplicative model
A model in which the joint effect of two or more factors is the product of their effects. For example, if one factor multiplies risk by a and a second factor by b, the combined effect of the two factors is a x b. See also additive model.

N

Negative study
A term used to refer to a study that does not have "statistically significant" (positive) results indicating a beneficial effect of the intervention being studied. The  term can generate confusion because it refers to both statistical significance and the direction of effect, studies often have multiple outcomes, the criteria for classifying studies as "negative" are not always clear and, in the case of studies of risk  or undesirable effects, "negative" studies are ones that do not show a harmful effect. See also Positive Study

Null hypothesis
The statistical hypothesis that one variable (e.g. whether or not a study participant was  allocated to receive an intervention) has no association with another variable or set of  variables (e.g. whether or not a study participant died), or that two or more population  distributions do not differ from one another. In simplest terms, the null hypothesis states that the results observed in a study are no different from what might have occurred  as a result of the play of chance.

Number needed to treat (NNT)
The number of patients who need to be treated to prevent one bad outcome. It is the inverse of the risk difference.