AFP Critical Appraisal & Stats Flashcards
How will you summarise what the paper is about?
Population = People with AF Intervention = Apixaban 5mg BD Control = Warfarin INR 2-3 Outcomes = Stroke or systemic embolism Key findings
What should you think about before summarising the paper?
QR
What is the research q and what is its relevance to clinical practise
What acronym do you use to assess internal validity?
Recruitment Allocation Maintenance Baseline Outcome (and was it blinded) Stats
What is internal validity?
The extent to which the observed results represent the truth in the population we are studying and, thus, are not due to methodological errors
How can we reduce selection bias?
Use consecutive recruitment rather than non-consecutive
Consider the recruitment location (e.g. GP vs hospital)
What types of recruitment are there?
Consecutive vs non-consecutive
Single centre vs multicentre
Types of allocation?
Randomised vs non randomised
Allocation blinded vs open label
Letters/Packets vs automated voice recognition system (electronic)
Block randomisation vs whole group randomisation
Cluster randomisation vs whole group randomisation
What is block randomisation?
Wait for X number of study participants and then say 50% go to each arm
Throughout the whole study there are equal numbers in both arms
Helps to mitigate temporal trends
What is cluster randomisation? Disadvantages?
Used in multicentre studies
Used when you can’t deliver different interventions in one place (e.g. poster in a GP practice)
Needs higher numbers of people
More difficult to achieve balance across groups
Benefit of randomisation?
Reduces risk of confounders
Why does blinding help?
Reduces risk of placebo effect
Reduces bias in data analysis
Benefit of Electronic voice recognition system?
Minimizes risk of tampering
What is study maintenance and what is usually aimed for?
Maintenance is the drop out rate from a study
Usually aim for <20% but ideally <10%
What bias is created if there is a poor maintenance?
Attrition bias - causes study to be under powered
Types of blinded outcome?
Single: patient only
Double: patient and physicians interacting
Triple: patient, physicians, outcome extractors
How can you blind if the outcome can’t be blinded e.g. surgical?
Use PROBE design
Prospective Randomised Open label Blinded endpoint adjudication
Whoever reviews the outcomes doesn’t know which arm of study the patient is in (e.g. radiologist when looking at stroke tx)
What do you need to look at in baseline?
Baseline characteristics of population and are they matched?
Besides blinding what else do you need to consider with outcome?
Is the choice of outcome important, adequate and important to patients?
Define power
the likelihood of detecting a difference when it exists (avoiding Type 2 error)
What 4 things should we look at when analysing the stats of a paper?
Power calculation and according recruitment target
Statistical models used for outcome measure (binary, continuous, time-dependent)
Effect size and significance level
Absolute risk reduction / Relative risk reduction / NNT
What 3 things does power calculation depend on?
- How many events are expected over follow-up time (incidence x time) - increase sample size
- Expected improvement (RRR of 50%) - increase effect size
- What probability you want to detect the difference when it exists (80%?) - increase precision of measurement
If a study is negative then think about the power!
Define p value
probability that the association detected has arisen by chance
How to strengthen the p value?
Size of association
Size of cohort
What test do you use to calculate difference between 2 groups with categorical variables when not adjusted? E.g. stroke vs no stroke
Chi-squared