Intended for healthcare professionals

Rapid response to:

Education And Debate

CONSORT statement: extension to cluster randomised trials

BMJ 2004; 328 doi: https://doi.org/10.1136/bmj.328.7441.702 (Published 18 March 2004) Cite this as: BMJ 2004;328:702

Rapid Response:

Reporting of cluster randomised trials when used in meta-analyses

Editor - when followed, the new CONSORT statement will greatly
facilitate the production of accurate estimates of effect for use in meta-
analyses. To date, coefficients of intracluster correlation (ICC) and
exact cluster sizes have not been reported systematically. At present, it
is common practice to 'borrow' suitable ICCs from other studies when
calculating summary statistics. Since the ICC can be affected by many
factors (for example, type of outcome, population, cluster size), this
practice is likely to have led to imprecision in some meta-analyses. The
guidance in the new CONSORT statement to report ICCs for all primary
outcomes is therefore very welcome.

However, the statement might also be the appropriate place to address
an additional source of imprecision. Some meta-analyses are based on
standardised mean differences and therefore require knowledge of the
standard deviation. Calculating this value from data presented from
cluster randomised trials presents particular challenges. For example,
sometimes studies report the standard deviation between clusters;
sometimes the standard error; and sometimes the standard error after
adjusting for baseline values. It is possible to calculate a standard
deviation from these data, but often additional information is required
(for example, the design effect used when the original standard error was
computed, or the multiple regression coefficient when baseline values were
taken into account). Thus, along with an ICC for each continuous outcome,
the production of accurate estimates of effect would be facilitated
greatly by the presentation of standard deviations (between individuals,
not clusters) too.

The new CONSORT statement is a significant step forward in ensuring
that cluster randomised trials are well reported, and, with a small
addition, it could be even more useful for future meta-analyses.

Competing interests:
None declared

Competing interests: No competing interests

26 March 2004
James M Thomas
Research Officer
Ian White
EPPI-Centre, Social Science Research Unit, Institute of Education, 18 Woburn Square, London, WC1H0NR