M

Masking
See blinding.

Matching
[In a case-control study:] Choosing one or more controls with particular matching attributes for each case. Researchers match cases and controls according to
particular variables that are thought to be important, such as age and sex. 

Mean
The average value, calculated by adding all the observations and dividing by the number of observations.  Also called arithmetic mean.

Mean difference
[In meta-analysis:] A method used to combine measures on continuous scales (such as weight), where the mean, standard deviation and sample size in each group are known. The weight given to the difference in means from each study (e.g. how much influence each study has on the overall results of the meta-analysis) is determined by the precision of its estimate of effect and, in the statistical software in RevMan and the Cochrane Database of Systematic Reviews, is equal to the inverse of the variance. This method assumes that all of the trials have measured the outcome on the same scale.  See also standardised mean difference.  (Also called WMD, weighted mean difference.)

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.

Meta-analysis
The use of statistical techniques in a systematic review to integrate the results of included studies. Sometimes misused as a synonym for systematic reviews, where the review includes a meta-analysis.

Meta-regression
[In meta-analysis:] A technique used to explore the relationship between study characteristics (e.g. concealment of allocation, baseline risk, timing of the intervention) and study results (the magnitude of effect observed in each study) in a systematic review. See also logistic regression.

Methodological quality 
See internal validity, bias prevention.

Minimisation
A method of allocation used to provide comparison groups that are closely similar for several variables. The next participant is assessed with regard to several characteristics, and assigned to the treatment group that has so far had fewer such people assigned to it. It can be done with a component of randomisation,
where the chance of allocation to the group with fewer similar participants is less than one.  Minimisation is best performed centrally with the aid of a computer program to ensure concealment of allocation. 

Morbidity
Illness or harm.  See also co-morbidity.

Mortality
Death.

Multi-arm trial
A trial with more than two arms.

Multicentre trial
A trial conducted at several geographical sites. Trials are sometimes conducted among several collaborating institutions, rather than at a single institution -  particularly when very large numbers of participants  are needed.

Multivariate analsis
Measuring the impact of more than one variable at a time while analysing a set of data, e.g. looking at the impact of age, sex, and occupation on a particular  outcome. Performed using regression analysis.