Epidemiology

Application of the scientific method to the study of disease or ill health and its alleviation amongst defined population.


Prevalence - the number of cases at any one point of time

Incidence - the number of new cases over a set period of time


Population estimates

  • UK population is estimated using census data

  • Births are added, deaths are subtracted, allowances for migration


Crude birth rate is the ratio of live births in a year and the total estimated populations in the same year.


Estimated population growth uses crude death/birth rates


Risk

  • Disease distributions are rarely random

  • Relative risk - the ratio of the probability of disease occurring in a subpopulation compared to another

  • e.g. relative risk of smoking is 1.5 if early mortality due to CVD being 30% risk for smokers and 20% risk for non-smokers


Relative risk should be interpreted with absolute risk


To avoid bias, our samples need to be representative of the population where every member has an equal probability of being sampled


Confounding - other factors can influence the relative risks and the outcome of interest


Reducing Confounding

  • Case-control design

  • Statistical control

  • Compare like for like


CCD:

Cases in subpopulations are matched to control cases in other subpopulations using specific criteria

Cases - patients with a specific outcome

Control - find a control without a specific outcome with similar characteristics

Test all cases and controls for the outcome


Statistical: using analysis techniques to statistically adjust for all dimensions. Save the relevant available. Usually longitudinal

e.g. Framingham Heart Disease Study

Problem: the size of the sample required for appropriate analysis.

Effort and cost are required to collect relevant data.


Simpsons PAradox

Comparison of 2 treatments for kidney stones

A works better on both small and large stones

B works better oevrall

Stone size is a confounding variable


Randomised control trails are the best for demonstrating causality. It can’t be used for ethical or logistic reasons often.


Prospective (longitudinal)

  • Events can’t cause other events that occur prior to them

  • Prospective designs address this reverse causality by measuring variables at different time points to discover sequence.


There is always the possibility that an unmeasured variable may be the source of correlation.


Controlled means that you intervene systematically in one group but not another. It means the other group is treated exactly the same (except they do not receive treatment)

Randomisation means you allocate people randomly to the control or treatment. Everone has an equal chance of being selected. Random allocation removes pre-existing differences as there’s no bias.