# Epidemiology

Screening is the application of a test to asymptomatic individuals to assess the likelihood that an individual has a particular disease.

Surveillance of disease is used to look for changes in trends or distribution of the disease.  Prevalence = Prevalencestart + Incidence Time

(when population and disorder stable)

A risk factor is an exposure or attribute that increases the probability of a disease.

DiseaseNo Disease
Exposureab
No Exposurecd

The absolute risk is always between zero and 1 and the relative risk is always larger than zero.

Absolute Risk Exposed Individuals: Absolute Risk Non Exposed Individuals: Relative Risk:

The relative risk is the risk of developing the disease that is due to an exposure in the exposed group Odds Ratio:

The odds ratio is the ratio of the odds of a disease occurring among exposed individuals to that of it occurring in unexposed individuals. An odds ratio or relative risk greater than 1 indicates the exposure is a risk factor for developing the disease. If the odds ratio or relative risk is 1, the exposure has no effect on the disease; if they are less than 1, the exposure protects against disease.

In R / JGR console 1:

library(epiR)
mat<-matrix(c(a,c,b,d),ncol=2) {enter values!}
mat
[,1] [,2]
[1,]   a  b
[2,]   c  d
epi.2by2(mat)
Outcome +    Outcome –      Total        Inc risk *        Odds
Exposed +
Exposed –
Total

Point estimates and 95 % CIs:
———————————————————
Inc risk ratio
Odds ratio
Attrib risk *
Attrib risk in population *
Attrib fraction in exposed (%)
Attrib fraction in population (%)
———————————————————
* Cases per 100 population units

The package epiR 1 should be installed.

The odds ratio provided is the maximum likelihood estimate that is different from the cross product ratio. To obtain the cross product ratio :

summary(epi.tests(mat))
est      lower      upper
aprev
tprev
se
sp
diag.acc
diag.or  {This is the cross product (diagnostic) odds ratio}
nnd
youden
ppv
npv
plr
nlr

 

 1. Stevenson M, Nunes T, Heuer C, Marshall J, Sanchez J, Thornton R, et al. epiR: Tools for the Analysis of Epidemiological Data [Internet]. 2015. (R package). Available from: http://cran.r-project.org/package=epiR