Epidemiology

Cumulative Incidence:

the proportion of people who develop a disease during a specified time period

CI=number who develop the disease in the time period

      number at risk of developing the disease at the start of the period

measured in x% per time period

A thousand healthy women were followed for 8 years and 15 developed high blood pressure

CI = 15/1000 per 8 years, or 1.5% in 8 years

Incidence Rate:

measures how quickly people are developing a disease

IR= number who develop the disease in the time period

       number of person years at risk of developing the disease

measured in x per y person years

in a community with a population of 50,000 people, 27 developed diabetes in a one year period.

IR = 27/(50,000 x 1)  = 5.4 per 10,000 person years.

a large group of elderly men was followed for a total of 5000 person years and 75 men had a stroke during the duration of the study.

IR = 75/5000   = 1.5 per 100 person years.

two thousand women were given a free health check and 100 were found to have high blood pressure. how many women were 'at risk'of developing high blood pressure?

IR=100 = 1/20

     2000

Point Prevalence:

the proportion of a population who have the disease at a given point in time.

PP = number who have the disease at a given point in time

         total number in population at a given point in time

measured in x% or x per 1000 population

prevalence = incidence x disease duration

two thousand women were given a health check and 100 had high blood pressure.

Prevalence = 100/2000 = 5%

on January 1st 2003 2 people were sick out of nine people in the group.

Prevalence = 2/9 = 22%

Relative Risk:

how likely is it that someone with the exposure will develop the outcome, compared to someone who is not exposed.

rate ratio = incidence rate exposed              risk ratio = cumulative incidence exposed

                   incidence rate unexposed                        cumulative incidence unexposed

Risk Difference:

a measure of the amount of disease that can be attributed to the exposure.

RD = incidence rate exposed  -  incidence rate unexposed

RD = cumulative incidence exposed  -  cumulative incidence unexposed

Odds Ratio:

an estimate of the relative risk when the outcome is rare in the population

(case-control studies)

OR = exposed cases/ unexposed cases

          exposed controls/ unexposed controls