what is a sample?
- a relatively small number of observations (or patients) from which we try to describe the whole population that the sample was taken - usually random
what is typically calculated from a sample?
- mean - confidence intervals to describe the range within which we think the population mean lies
what is a null hypothesis?
the hypothesis that there is no significant difference between study populations, any differences being due to chance
when is a null hypothesis rejected?
when p < 0.05
what does a confidence interval of 95% show?
that if the sampling was repeated 100 times, the result observed would fall within the CI range in 95/100 samples
what are 3 examples of measures of association?
- relative risk
- attributable risk
- odds ratio
what are odds?
another way of expressing probability - the odds of exposure is the number of people who have been exposed/the number of people who have not been exposed
what is the mathematical relationship between odds and probability?
odds = probability/(1-probability)
what is relative risk?
a measure of association between an exposure and disease
what does a relative risk value of 1 show?
the incidence of disease in the exposed and unexposed is the same so the data shows no association between the exposure and the disease
what does a relative risk value >1 show?
positive association or an increased risk among those that are exposed
what does a relative risk value <1 show?
inverse association or decreased risk among those that are exposed (exposure is protective)
what is attributable risk?
a measure of exposure effect that indicates on an absolute scale how much greater the frequency of disease in the exposed group is compared with the unexposed, assuming the relationship between exposure and disease is causal
how do you calculate relative risk?
incidence in the exposed group/the incidence in the unexposed group
how do you calculate attributable risk?
(incidence in the exposed - incidence in the unexposed)/100
draw a grid that can be used to calculate risk
what is attributable risk especially useful for evaluating?
the impact of introduction/removal of risk factors
what is odds ratio?
the odds ratio (of exposure) is the ratio between 2 odds
how do you calculate odds ratio?
odds of exposure in the diseased (cases)/odds od exposure in the disease-free (controls)
what does an odds ratio of 1 show?
exposure is no more likely in the cases than controls (therefore exposure has no effect on case/control status)
what does an odds ratio >1 show?
exposure is more likely in the case group (so exposure may increase the risk of disease)
what does an odds ratio <1 show?
exposure is less likely in the case group (therefore exposure may have a protective effect)
what's the difference between relative risk and odds ratio?
- relative risk can be calculated from cohort studies as the incidence of disease in the exposed and unexposed is known
- odds of exposure (and therefore odds ratio) can be calculated from case-control studies where the subjects are selected on the basis of their disease status rather than exposure
list 3 methods for dealing with confounding factors
how is stratification a method for dealing with confounding factors?
controls the effect of confounders at the analysis stage as risks are calculated separately for each category of confounding variable (e.g. age, sex etc)
how is standardisation a method for dealing with confounding factors?
controls the effect of confounders at the analysis stage as it is used to produce a Standardised Mortality Ratio
how is regression a method for dealing with confounding factors?
controls the effect of confouders at the analysis stage as statistical modelling is used to control for one/many confounding variables