Experimental study - week 5 Flashcards
(42 cards)
What are experimental studies?
Experimental or therapeutic studies involve an active attempt to change a variable in a group (e.g. disease, behaviour) via an intervention.
The effects of an intervention are measured by comparing the outcome incidence in the group who have experienced the intervention (experimental group) with that of the group you haven’t intervened with (control group).
Randomised control trial
In a randomised control trial (RCT) patients are randomly allocated to a treatment group (exposed group) or a control group (unexposed group).
‘Gold standard’ for studying cause and effect
• Critical aspects are random allocation and allocation concealment (to ensure there is
no confounding)
STRENGTHS of randomized control trials
The ‘gold standard’ epidemiological studies
Lower risk of bias and confounding than any other epidemiological study
Provide strong evidence of causal relationships
Can be used to study multiple outcomes
Measure the incidence rate of an outcome
LIMITATIONS of randomized control trials
often expensive Ethical issues Generalisability needs to be carefully assessed Require large study group Long follow-up period
Is RCT a prospective study
Since the patients in each group are followed prospectively, the study is often called a prospective study.
Randomisation
Procedure
1- Before admission to a study, every participant must be suitable for either treatment
2- Randomisation must occur AFTER commitment to participate as bias can arise if patients/doctors know the treatment to be allocated and then decide to withdraw
3- Participants allocated to treatment groups by chance (easily achieved through the use of random number generators) – all participants have an equal chance of being allocated to treatment or control
4- All participants must be followed-up and included in the study regardless of later compliance
Benefit of Randomisation
BENEFITS
Produces groups that are not systematically different with regard to known and unknown prognostic factors
Any difference between groups that arise after randomisation could be due to consequences of the randomised treatment assignment
Permits a valid analysis
Permutation test is justified by randomisation
Standard analyses are valid approximations of the correct permutation test
Eliminates selection bias e.g. doctors selecting sicker patients to be in the new treatment group
Allocation concealment
Those responsible for recruiting participants must not know what groups they will be allocated to. This often requires a third party to maintain a private list of consenting participants and their random allocation.
Doctors may manipulate the allocation process for their patients which undermines the validity of the study. Allocation concealment prevents this from occurring.
• Simple random allocation
• Stratified allocation
Blinding
Blinding is the most effective way of eliminating systematic error.
However, in some case it is not possible to have blinding as it will be obvious a patient is in the treatment group i.e. if there are pronounced side effects of the drug or the intervention is considerably different to the control (e.g no exercise vs exercise).
If an emergency occurs it may be necessary to divulge the treatment the patients were receiving. This would often require a third party to hold the code and “break” it if needed.
What is the main purpose of randomisation?
a. To control for confounding by distributing known and unknown confounders equally
b. To ensure that participants adhere to the study protocol
c. To prevent the placebo effect by ensuring participants do not know if they are taking the placebo of the intervention
d. To prevent researchers from manipulating their test subjects in order to have fabourable results
To control for confounding by distributing known and unknown confounders equally
An advantage of a double-blinded randomised control trial is:
Creates a group of people representative of the target population
Equally distributes known and unknown confounders
Encourages participants to adhere to the study protocol
Prevents the bias that arises from researchers being able to influence collected data with the knowledge of allocated groups
Prevents the bias that arises from researchers being able to influence collected data with the knowledge of allocated groups
What is random selection?
Randomly selecting people from the target population to form the sample population
Randomly assigning subjects into control and intervention groups
A term synonymous with random allocation
Randomly selecting people from the target population to form the sample population
blinding why ?
Process where either, or both, the participant and the investigator remain
unaware of which trial group the participants are in
• Why?
• Performance bias
• refers to systematic differences between groups in the care that is provided, or in
exposure to factors other than the interventions of interest.
Easier to do with drug trials in which can use a placebo
• Must be same colour, shape, size…
Placebo or control group
• an inert substance or procedure or standard care
• used in clinical trials as a baseline
• Placebo effect or response is believing that the treatment will work
i.e. that a fake treatment is the real thing
Different types of RCTs
Open RCT
• Everybody involved knows which intervention is given to each patient
Single-blind
• One group of individuals does not know the identity of the intervention
given to participants
Double-blind
• Two groups of individuals do not know the identity of the intervention
given to participants
• Performance bias and detection bias are minimised
performance bias and detection bias
Performance bias: refers to differences that occur due to knowledge of interventions allocation, in
either the researcher or the participant
Detection bias: refers to systematic differences between groups in how outcomes are determined
Cross-over trial
A crossover design is a repeated measurements design such that each experimental unit (patient) receives different treatments during the different time periods, i.e., the patients cross over from one treatment to another during the course of the trial
What are field trials?
As the name suggests field trials are trials that are conducted in the ‘field’. They are aimed at disease-free people in the population – participants do not have the disease of interest but are at risk.
Their aim is to evaluate interventions (preventive studies or strategies) to reduce specific exposures. Examples could include:
Education methods (e.g. skin cancer education) Training procedures (e.g. workplace safety) Other
Cross over trial advantage and limitation
Advantages • Patients serve as own controls • Sample size Limitations • Order of events? • Not feasible • Not ethical
N of 1 trial
- A single case is the participant of the trial
* Also can be an open ‘before and after’ trial
Advantage and disadvantage of N of 1 trial
Advantages • Can confirm causality and effectiveness • Useful for piloting treatment Limitations • Generalisability
Cluster randomised trials
• Cluster means that the unit of randomisation is not the individual.
• Often used when a study is done in the community and there is risk of
“contamination”
RCT CANNOT BE USED FOR
It may not be possible to do an RCT so information from other study
designs must be used
• We cannot use an RCT to establish what patterns of blood lipids predispose people
to developing coronary heart disease - this will require a cohort study
• We can use RCTs to answer simple questions
Principles of systematic reviews
Overview of medical literature with reproducible objectives and
methodology
• Validity of individual trials assessed by specific criteria
• Summary of results via qualitative and quantitative methodology
• e.g. meta-analysis
