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 ~ Incidence x Mean disease duration
(when population and disorder stable)
A risk factor is an exposure or attribute that increases the probability of a disease.
| Disease | No Disease | |
|---|---|---|
| Exposure | a | b |
| No Exposure | c | d |
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 the R console1:
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 epiR2 should be installed.
Please note, the maximum likelihood odds ratio is a different estimate than 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