Excellent website on Research Design by CEBM The Centre for Evidence-Based Medicine (University of Oxford)

Content 1

Techniques such as pilot studies, questionnaire design, field observations, interviews, focus groups and analysis of transcripts of narrative material, ethnography and observation, action research, case study

Consensus methods such as Delphi or nominal groups

Content 2

The hierarchy of design and the advantages and disadvantages of study designs including:
Systematic reviews and meta-analysis
Experimental: randomised controlled double blind or non-randomised controlled trial or quasi-experimental
Observational: cohort (prospective, retrospective), case-control, cross-sectional

The “hierarchy of evidence”

Hierarchy of strength of evidence used:

Ia – evidence from systematic reviews or meta-analysis of randomised controlled trials

Ib – evidence from at least one randomised controlled trial

IIa – evidence from at least one controlled study without randomisation

IIb – evidence from at least one other type of quasi-experimental study

III – evidence from non-experimental descriptive studies, such as case-control studies

IV – evidence from expert committee reports or opinions or clinical experience of respected authorities

OBSERVATIONAL (NON-EXPERIMENTAL STUDY DESIGNS)

Cohort studies
Cohort studies are longitudinal studies that follow a large cohort of people over a long period of time
Explore the risk of developing an outcome, over time, to a specific exposure.
These studies are useful to assess the effect of genetic variants, rare exposures or exposures that take a long time to produce disease.
Cohort studies are also expensive to conduct and are reliant on the right exposures being measured and ability to follow-up a large proportion of the cohort. 

Case-Control studies
This is an observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute.
Case-­‐control studies are often used to identify factors that may contribute to a medical condition by comparing subjects who have that condition/disease (the 'cases') with patients who do not have the condition/disease but are otherwise similar (the 'controls').
Usually less expensive
Sampling from source population
Can usually calculate only the ratio of incidence rates or risks
Convenient for studying many exposures

Cross-­‐sectional studies
A cross-­‐sectional study is a descriptive study in which disease and exposure status are measured simultaneously in a given population.
Cross-­‐sectional studies can be thought of as providing a "snapshot" of the frequency and characteristics of a disease in a population at a particular point in time.
This type of data can be used to assess the prevalence of acute or chronic conditions in a population.
However, since exposure and disease status are measured at the same point in time, it may not be possible to distinguish whether the exposure preceded or followed the disease, and thus cause and effect relationships are not certain.

EXPERIMENTAL STUDIES (RANDOMISED CONTROLLED TRIALS)

These are cohort studies where allocation to treatment and control groups is achieved by a random process.

Random allocation is helpful for reducing ‘selection bias’ and ‘allocation bias’, especially when combined with allocation concealment.
Because they are experiments, randomized trials can also employ blinding of participants and caregivers, which reduces ‘performance bias’.