Note: Users need to be very clear about the way in which the terms ‘group’ and ‘cluster’ are used in these tables. Table 13.2.a only refers to groups, which is used in its conventional sense to mean a number of individual participants. With the exception of allocation on the basis of outcome, ‘group’ can be interpreted synonymously with ‘intervention group’. Table 13.2.b refers to both clusters and groups. In this table, ‘clusters’ are typically an organizational entity such as a family health practice, or administrative area, not an individual. As in Table 13.2.a, ‘group’ is synonymous with ‘intervention group’ and is used to describe a collection of allocated units, but in Table 13.2.b these units are clusters rather than individuals. Furthermore, although individuals are nested in clusters, a cluster does not necessarily represent a fixed collection of individuals. For instance, in cluster-allocated studies, clusters are often studied at two or more time-points (periods) with different collections of individuals contributing to the data collected at each time-point.

Was there a comparison?

Typically, researchers compare two or more groups that receive different interventions; the groups may be studied over the same time period, or over different time periods (see below). Sometimes researchers compare outcomes in just one group but at two time-points. It is also possible that researchers may have done both, i.e. studying two or more groups and measuring outcomes at more than one time-point.

Were participants/clusters allocated to groups by?

These items aim to describe how groups were formed. None will apply if the study does not compare two or more groups of subjects. The information is often not reported or is difficult to find in a paper. The items provided cover the main ways in which groups may be formed. More than one option may apply to a single study, although some options are mutually exclusive (i.e. a study is either randomized or not).

Randomization: Allocation was carried out on the basis of truly random sequence. Such studies are covered by the standard guidance elsewhere in this Handbook. Check carefully whether allocation was adequately concealed until subjects were definitively recruited.

Quasi-randomization: Allocation was done on the basis of a pseudo-random sequence, e.g. odd/even hospital number or date of birth, alternation. Note: when such methods are used, the problem is that allocation is rarely concealed. These studies are often included in systematic reviews that only include randomized trials, using assessment of the risk of bias to distinguish them from properly randomized trials.

By other action of researchers: This is a catch-all category and further details should be noted if the researchers report them. Allocation happened as the result of some decision or system applied by the researchers. For example, subjects managed in particular ‘units’ of provision (e.g. wards, general practices) were ‘chosen’ to receive the intervention and subjects managed in other units to receive the control intervention.

Time differences: Recruitment to groups did not occur contemporaneously. For example, in a historically controlled study subjects in the control group are typically recruited earlier in time than subjects in the intervention group; the intervention is then introduced and subjects receiving the intervention are recruited. Both groups are usually recruited in the same setting. If the design was under the control of the researchers, both this option and ‘other action of researchers’ must be ticked for a single study. If the design ‘came about’ by the introduction of a new intervention, both this option and ‘treatment decisions’ must be ticked for a single study.

Location differences: Two or more groups in different geographic areas were compared, and the choice of which area(s) received the intervention and control interventions was not made randomly. So, both this option and ‘other action of researchers’ could be ticked for a single study.

Treatment decisions: Intervention and control groups were formed by naturally occurring variation in treatment decisions. This option is intended to reflect treatment decisions taken mainly by the clinicians responsible; the following option is intended to reflect treatment decisions made mainly on the basis of subjects’ preferences. If treatment preferences are uniform for particular provider ‘units’, or switch over time, both this option and ‘location’ or ‘time’ differences should be ticked.

Patient preferences: Intervention and control groups were formed by naturally occurring variation in patients’ preferences. This option is intended to reflect treatment decisions made mainly on the basis of subjects’ preferences; the previous option is intended to reflect treatment decisions taken mainly by the clinicians responsible.

On the basis of outcome: A group of people who experienced a particular outcome of interest were compared with a group of people who did not, i.e. a case-control study. Note: this option should be ticked for papers that report analyses of multiple risk factors for a particular outcome in a large series of subjects, i.e. in which the total study population is divided into those who experienced the outcome and those who did not. These studies are much closer to nested case-control studies than cohort studies, even when longitudinal data are collected prospectively for consecutive patients.

Additional options for cluster-allocated studies.

Location differences: see above.

Policy/public health decisions: Intervention and control groups were formed by decisions made by people with the responsibility for implementing policies about public health or service provision. Where such decisions are coincident with clusters, or where such people are the researchers themselves, this item overlaps with ‘other action of researchers’ and ‘cluster preferences’.

Cluster preferences: Intervention and control groups were formed by naturally occurring variation in the preferences of clusters, e.g. preferences made collectively or individually at the level of the cluster entity.

Which parts of the study were prospective?

These items aim to describe which parts of the study were conducted prospectively. In a randomized controlled trial, all four of these items would be prospective. For NRS it is also possible that all four are prospective, although inadequate detail may be presented to discern this, particularly for generation of hypotheses. In some cohort studies, participants may be identified, and have been allocated to treatment retrospectively, but outcomes are ascertained prospectively.

On what variables was comparability of groups assessed?

These questions should identify ‘before-and-after’ studies. Baseline assessment of outcome variables is particularly useful when outcomes are measured on continuous scales, e.g. health status or quality of life.

Response options

Try to use only ‘Yes’, ‘No’ and ‘Can’t tell’ response options. ‘N/a’ should be used if a study does not report a comparison between groups.