Critical appraisal Flashcards
Randomisation
Purpose is to try and ensure that any characteristics of the sample population that may affect the results ( confounders) are distributed equally between the two study groups, and avoiding selection bias
Tools for randomisation:
centralized computer randomization (with contact by phone or computer) is ideal and often used in multicentre trials.
smaller trials may use an independent person (eg the hospital pharmacy to manage the randomization
Stratified randomisation
With powerful confounders ( eg age, sex) , patients can first be split, or stratified , into different groups before randomisation, so there will be the same number of patients with and without the confounder ( young and old, male and female) in each arm of the trial
allocation concealment
whether the person recruiting the patient to the trial could know or anticipate the group allocation that patient would receive, preventing selection bias
Prevents clinicians predicting which group patients would be in before recruiting them to the trial
Blinding
When some of the partiticapits of a trial ( patient/clinicians/researchers) are prevented from knowing certain information that may lead to conscious or subconscious bias
Blinding advantages
- Prevents observer bias = form of reactivity in which researcher’s cognitive bias causes them to subconsciously influence the participants of experiment = could influence extra quality of care to these patients
- Or confirmation bias = see results that aren’t there
- Expectation bias (Pygmalion effect) = Observers may subconsciously measure or report data in a way that favours the expected study outcome
Also prevents placebo effect – or reduces it
double blinded trial
both patients and investigators are unaware of treatment allocation
when blinding is best to use
outcome is subjective (eg measurement of symptoms or function)
or if outcome measurement is based on patient self report
instances when blinding is not needed
if outcome is objective eg death
sometimes impossible to achieve eg in a trial involving physiotherapy , they will know whether or not they have received it
in a trial involving warfarin ,clinical cannot be blinded due to safety reasons
Confounding
distortion (or potential for distortion) of association between outcome and exposure
by third factor
which has an association with both exposure and outcome
common causes of confounding.
Confounding occurs when there is a non random distribution of risk factors in the populations. Age, sex and social class are common causes
How to control confounding factors
In the design stage of an experiment, confounding can be controlled by randomisation which aims to produce an even amount of potential risk factors in two populations.
In the analysis stage of an experiment, confounding can be controlled for by stratification.
Study least subject to bias
RCT - the groups are likely to be similar with respect to known and unknown determinants of outcome therefore we can be more confident that any observed differences in outcome are due to the intervention.
‘intention to treat’ analysis
Statistical analysis of data from subjects according to the group to which they were assigned despite noncompliance with the study protocol
‘per protocol’ analysis
An analysis of patient outcomes based only on those subjects who completed all aspects of the protocol. Also called on-treatment analysis.
Treatment fidelity
how accurately the intervention is reproduced from a protocol or model
Validity
describes how accurately a study, instrument, test or equivalent measures what it is supposed to.
Factors affecting validity
study size
inter-participant variability the use of different measuring instruments (instrumentality)
Certain biases such as attrition and selection bias
internal validity
how well the study was conducted, the degree to which the effects observed in an experiment are due to the independent variable and not confounds-true, accuracy
Threats to internal validity
Reliability of measurement instruments Regression towards the mean Sampling Experimental mortality Instrument obtrusiveness Maturation Measurement instrument learning
external validity
extent to which we can generalise findings to real-world settings-useful, generalisability
Threats to external validity:
Representativeness of the sample
Reactive effects of setting (is the research setting artificial)
Effect of testing (if a pre-test was used in the study that will not be used in the real world this may affect outcomes)
Multiple treatment inference (this refers to study’s in which subject receive more than one treatment, the effects of multiple treatments may interact)
Reliability
is the extent to which an experiment, test, or any measuring procedure yields the same result on repeated trials.The higher the reliability the more likely you are to obtain similar results if the study was repeated.Reliability does not ensure accuracy.
Cohort study
sample that has been exposed to a certain exposure and follow that sample to observe the outcome. Cohort studies can be retrospective or prospective. used for prognosis and studying rare exposures. But if uncommon event cohort study would have to be unfeasibly large to answer the study question