Ideas on Study Design

A selection of resources about identifying study designs:

Study Design Algorithm

This is a nice flowchart that describes the major study designs and includes questions about the purpose of the study to guide you through.

From the Center for Evidence Based Medicine

Dr. Rod Jackson on Study Designs

A description of basic clinical research designs from Professor Rod Jackson from the University of Auckland in NZ:

Epidemiological studies can be classified in several ways.

• First, studies can be classified as either experimental or observational.

  • In an experiment, the study investigator controls the allocation of the exposure (or exposures) of interest to the participants. In an experiment the exposure is always an 'intervention' which is usually compared with another intervention or placebo. As you know, in the ideal experiment, allocation of interventions to different arms of the study is done using a randomisation process so each participant has an equal chance of being in any of the study arms (i.e. a RCT).

  • The other type of study is an observational study in which the investigator 'observes' (rather than controls) what each participant is exposed to. For example the investigator observes whether the participants are taking a particular drug or not or, say, whether they are smokers or non smokers. The participants are then allocated to different arms of the study based on their exposure status.

• Second, studies are then classified by whether the participants are followed up over time to determine the study outcomes (a follow-up or longitudinal study) or whether whether both exposure status and outcome status is measured at the same time (a cross-sectional study).

  • All experiments are longitudinal because if the experimenters allocate participants to exposure, they need to follow them for a period of time to determine the effect of the exposures on outcomes (unless the effect of the exposure on outcomes is instantaneous which is theoretically possible but unlikely).

  • In contrast observational studies can be longitudinal (i.e. Cohort studies) or cross-sectional (e.g. Surveys) or a combination of both. Observational studies can be both longitudinal and cross-section because the investigator only observes if participants have been exposed to the exposure of interest. Therefore participants have almost always been exposed (or not exposed) to the exposure of interest for some time before the study starts.

• Third, most studies have at least 2 arms - exposure [treatment] and comparison groups - but both experiments and observational studies could just have one arm (i.e. no comparison arm). Small groups of patients in 'one arm' studies are typically called case series. They are experiments if the investigator controls the 'exposure' and observational studies if the exposure allocation is not part of a study and the investigator observes what participants have been exposed to. However even most small case series can be divided into smaller subgroups (as in some of the studies you describe) so the definition of a case series is rather lose but generally describes one smallish group of patients with a similar condition and similar treatment who are followed for a period of time.

• While a cohort study classification is generally only given to longitudinal observational studies, RCTs could be considered as cohort studies in which exposure has been randomly allocated by the investigator.

• A case-control study is a sub-type of a observational cohort study. You may have heard about nested case-control studies. The standard case control study is simply nested in a virtual cohort study.

I hope these comments are of some help