Alfred Saturday Statistics Flashcards
Nominal vs ordinal data
Both categorical data but ordinal can be ordered/ranked (still have to use non-parametric analysis methods)
What is the explanatory variable
The independent variable/exposure (usually the x-axis)
Aim of multivariable analysis
To determine the unique contribution of various factors to a single event/outcome
Parametric vs non parametric data
Parametric data
- Based on the following assumptions
- Continuous data
- Normal distribution in the population
- Population and sample have the same variance and standard deviation
- Better powered than non-parametric
Non-parametric data
- Distribution free (less assumption), does not require numerical parameters
What factors affect power?
- Difference in outcome rates
- Level of significance (p value)
- SD of population (smaller SD higher power)
- Sample size
How to calculate the probability of a type I and type II error?
Type I
- Alpha (designated p value)
- ALso false positive rate = 1-specificity
Type II
- 1-Beta (power)
- Also false negative = 1-sensitivity
How to calculate odds
rate of occurance / rate of non-occurance
(c.f. probability where is the rate of occurance / sample number)
What is the median ratio in survival analysis?
Better explains clinical effect
Ratio of duration of survival at 50% event mark
Self controlled case series
- When is it used
- What is the aim
- When you can’t suitable controls
- Assessing time of event compared to time of exposure
- Aims to estimate the relative incidence within periods of hypothesised excess risk due to exposure versus incidence of adverse effects during all other times
Often used with vaccines and assessing event rate during high risk time after vaccine (e.g. stroke and MI rates transient increased risk after exposure to herpes zoster)
What is a matched block design?
It is where subjects are put into pairs with closely matching characteristics. It can also be done as a “repeated measure design” where the 2 arms are repeat in 1 subject
The aim is to reduce confounders
How to reduce information bias/selection bias
Blinding
How to reduce the effect of confounders?
Randomisation
What bias occur in cross-over studies when the likelihood of crossing over is different in the 2 groups?
Selection bias - When the sick patients are less likely to cross over into the control group
What is ascertainment bias
Sampling bias
E.g. patients who have Alzheimer’s and people who are on statins are generally associated with the frequency of visiting the doctor
What does selection/ascertainment bias affect?
Generalisability of the study
