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=\frac{Number\; Of\;Existing\;Cases\;per\;Year}{Population\;at\;Risk}

Incidence=\frac{Number\; Of\;New\;Cases\;per\;Year}{Population\;at\;Risk}

Prevalence = Prevalencestart + Incidence \times 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:

ARexposure = \frac{a}{a+b}<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />

Absolute Risk Non Exposed Individuals:

ARnoexposure = \frac{c}{c+d}<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />

Relative Risk:

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

RR=\frac{a \over (a+b)}{ c \over (c+d)}

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.

OR=\frac{a \times d}{b \times c}

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