S2 Measurement and Study Designs Flashcards
What is the purpose of statistics?
To generalise to/infer about the population
How do we making inferences about the entire population?
Take a sample which is representative, unbiased and precise
What are the two types of error that can occur in a study that may influence results?
- Chance/random error - due to sampling variation, will reduce as sample size increases
- Bias/systematic error - difference between the true value and the expected value, doesn’t reduce as sample size increase
What are sources of bias?
- Selection bias
2. Information bias
What are some types of selection bias?
- Study sample not representative of whole population of interest
- Group selection within a study - groups in a study not comparable
- Healthy worker effect - people who work have a lower overall mortality than the general population
What are some types of information bias?
- Recall error - differences in recollection
- Observer/interviewer error - preconceived expectations/knowledge
- Measurement error - differences in measurement
- Misclassification - participants put into wrong group usually arises from observational or measurement error
What is prevalence?
The proportion of people who have a disease at a given point in time
Often reported as a proportion
What is incidence?
The number of new cases of a disease within a given timeframe
Often reported as a rate
When is someone no longer counted in prevalence?
If they die or are cured
If they are cured and have a regression, they are classed as a new case (incidence)
What is the incidence rate ratio?
Compares the incidence rate in one group to another
It is a relative measure between two groups (or more groups)
How do you calculate incidence rate ratio?
IRR = incidence rate in group A / incidence rate in group B
How do you calculate incidence rate?
Incidence rate = number of new cases / (sum of) patient time at risk = events per person per year
Why do you measure incidence rate as a rate (e.g. patient time at risk)?
Not all patients are at risk throughout the whole study/whole life e.g. if someone is dead they are no longer at risk
How do you calculate prevalence?
Prevalence = number of people with the disease / total population
Do ‘relative risk and risk ratio’ and
‘relative rate and rate ratio’ have the same meanings?
Yes
If the probability of an event is p, how do you calculate the odds of that event?
Odds = p / (1-p)
How do you calculate odds ratio?
Odds ratio = odds of group A / odds of group B
This is a relative comparison
In cohort study, with consistent follow-up, we can calculate odds ratio/relative risk estimates directly, what is the calculation?
Relative risk = absolute risk for group A / absolute risk for group B
What would be the ratio if there was no difference between the groups?
1
What is the calculation for risk difference?
Risk difference = absolute risk in group A - absolute risk of group B
What is an issue to be aware of when comparing groups? What do you do about it?
Confounding factors (things you can’t control)
Adjust for differences in known confounding factors - standardisation
What is risk difference?
The absolute difference in risk between one group compared to another
What is relative risk/risk ratio?
Describes the comparison of the probability of an Evernote occurring in one group compared to another group
What are confounding factors?
Factors that, when comparing groups, the association/effect between an exposure and outcome is distorted by the presence of another variable
