Deciding which study Deciding which study designs to include: RCTs, designs to include: RCTs,

Cohort studies, Case Cohort studies, Case studies, Interrupted Time-studies, Interrupted Time-

seriesseries

Roger ThomasRoger Thomas

Excluded studiesExcluded studies

Two reviewers independently read all titles Two reviewers independently read all titles and abstracts selected by the literature and abstracts selected by the literature review and exclude or exclude studiesreview and exclude or exclude studies

If the abstract does not provide enough If the abstract does not provide enough information, independently read full text of information, independently read full text of articlearticle

Record reasons why studies were excluded Record reasons why studies were excluded on a simple form with 3 or 4 categorieson a simple form with 3 or 4 categories

Included StudiesIncluded StudiesRecord details of included studies with a Record details of included studies with a

different form for each type of study designdifferent form for each type of study design RCT RCT

Cohort or Case studyCohort or Case study Interrupted Time SeriesInterrupted Time Series

On each form include details of your PICOS On each form include details of your PICOS information:information:

Participants (including exclusions)Participants (including exclusions) Interventions (including co-interventions)Interventions (including co-interventions) ComparisonsComparisons OutcomesOutcomes

Definition of an RCTDefinition of an RCT

An experiment in which two or more An experiment in which two or more interventions, possibly including a control interventions, possibly including a control intervention or no intervention, are intervention or no intervention, are compared by being randomly allocated to compared by being randomly allocated to participants. participants.

In most trials one intervention is assigned In most trials one intervention is assigned to each individual but sometimes to each individual but sometimes assignment is to defined groups of assignment is to defined groups of individuals (for example, in a household) individuals (for example, in a household) or interventions are assigned within or interventions are assigned within individuals (for example, in different individuals (for example, in different orders or to different parts of the body).orders or to different parts of the body).

Cluster-RCTs (C-RCTs)Cluster-RCTs (C-RCTs) If the intervention is given to a group, If the intervention is given to a group,

statistical analysis must adjust for the statistical analysis must adjust for the effects of clusteringeffects of clustering

The sample size is now the number of The sample size is now the number of groups, not the number of participantsgroups, not the number of participants

Whether the intervention caused the effect Whether the intervention caused the effect or whether group discussion (e.g. among or whether group discussion (e.g. among students in a class) also caused the effect students in a class) also caused the effect can be adjusted for by computing the can be adjusted for by computing the Intra Class Correlation (ICC) or Intra Class Correlation (ICC) or generalised estimating equations generalised estimating equations

You will need to collect this additional data You will need to collect this additional data on C-RCTson C-RCTs

Definition of a Cohort StudyDefinition of a Cohort Study An An observational studyobservational study in which a in which a

defined group of people (the cohort) is defined group of people (the cohort) is followed over time (cohors = a unit 600 followed over time (cohors = a unit 600 auxiliary horsemen in the Roman army)auxiliary horsemen in the Roman army)

The The outcomesoutcomes of people in subsets of this of people in subsets of this cohort are compared, to examine people cohort are compared, to examine people who were exposed or not exposed (or who were exposed or not exposed (or exposed at different levels) to a particular exposed at different levels) to a particular interventionintervention or other factor of interest. or other factor of interest.

Because subjects are not allocated by the Because subjects are not allocated by the investigator to different interventions or investigator to different interventions or other exposures, other exposures, adjusted analysisadjusted analysis is is usually required to minimise the influence usually required to minimise the influence of other factors (of other factors (unknown unknown confoundersconfounders).).

Definition of a Case Control StudyDefinition of a Case Control Study A study that compares people with a A study that compares people with a

specific disease or specific disease or outcomeoutcome of interest of interest (cases) to people from the same (cases) to people from the same populationpopulation without that disease or without that disease or outcome (outcome (controlscontrols), and which seeks to ), and which seeks to find associations between the outcome find associations between the outcome and prior exposure to particular risk and prior exposure to particular risk factors. factors.

This design is particularly useful where the This design is particularly useful where the outcome is rare and past exposure can be outcome is rare and past exposure can be reliably measured. reliably measured.

Case-control studies are usually Case-control studies are usually retrospectiveretrospective

Solutions to the matching problem Solutions to the matching problem for case control studiesfor case control studies

The problem is finding cases that can be The problem is finding cases that can be matched on more than one matched on more than one characteristics related to the outcome characteristics related to the outcome (e.g. age, gender, education, health (e.g. age, gender, education, health status)status)

2.2. Match each case to e.g. 4 controls so the Match each case to e.g. 4 controls so the problems in matching average out over 4 problems in matching average out over 4 casescases

3.3. Compute a “propensity score” combining Compute a “propensity score” combining many characteristics on which one wants many characteristics on which one wants to match, so that cases are matched to to match, so that cases are matched to the control with the propensity score the control with the propensity score closest to them closest to them

Definition of an Interrupted Time Definition of an Interrupted Time SeriesSeries

A research design that collects A research design that collects observations at multiple time points before observations at multiple time points before and after an and after an interventionintervention (interruption). (interruption).

The design attempts to detect whether the The design attempts to detect whether the intervention has had an effect significantly intervention has had an effect significantly greater than the underlying trend.greater than the underlying trend.

The Cochrane EPOC group requires 3 The Cochrane EPOC group requires 3 observations before and 3 after the observations before and 3 after the interventionintervention