Summary EPI4291
Association Measures
INCIDENCE
- The number of new cases in a given period in a specified population (time must be
specified)
- Measures the rapidity with which new cases are occurring in a population
- can be expressed:
o in absolute numbers
o in terms of cumulated incidence
o in terms of incidence density
Formula to go from IR to Ci
CI= 1-e^-(i x t)=
Cumulative incidence:
N of new cases
CI = x 10n
population at risk at beginning
CI 0= A/A+B Unexposed people
CI 1 = C/C+B Exposed people
CIt = A+B/N= 1−exp(−∑ Iι Δtι)
Proportion of people that develop disease during a certain time interval
Interpretation:
The changes to get sick when exposed is ...% in .. years
, Cumulative Incidence Rate
CIR= RR
Disease D+ No disease D- Total
Exposed E+ A B a+b
Non Exposed E- C D c+d
Total E F N
Incidence rate/ Incidence Density
IR= ID
ID= exposed/ population x Time
Exposed/ person-time
N of new cases
ID= x 10 n
total person−time of observation
ID= number of Incidence Cases/person years at risk = M1/L
IDe+ ¿ D 1=a /L 1
IDe-= D0 = b/Lo
Describes trends in diseases and evaluates impact of primary prevention programmes
E+ E- TOT
D+ A B Mt
PT L1 L0 L
The number of NEW cases per unite of population per unit of time.
IDR = incidence density ratio= ID1/ID0
IDD= incidence density difference= ID1-ID0
PREVALENCE
Prevalence= number of existing cases in the pop at a certain time/ number of persons in the
population at that certain time.
( people with disease at a specified time)
P= x 10n
( population at risk at the specified time)
I x ∆ t x ( N−P)
Proportion of people who HAVE the disease on a certain moment or in a certain period of
time.
Point Prevalence:
- Measures the frequency of disease at a given point in time
- Applies when the data has been collected at one point in time
, - P=C / N
o C= number of observed cases at time t
o N= population size at time t
Period prevalence
- Measures the frequency of disease over some time
- Applies when the data has been collected over a period of time
- P=C + I /N
o C= number of prevalent cases at the beginning of the time period
o I= number of incidence cases that develop during the period
o N= population size for this same time period
The relation between Prevalence and Incidence:
- Prevalence depends on:
Incidence (I). duration of the disease (D)
- Change in prevalence from one time period to another may be the result of changes
in Incidence rate, changes in the duration of disease or both
Case Fatality
Place in relation of number of deaths from a disease to the number of cases
- Reflects severity
- Can be expressed as:
o Proportion
o Ratio
o Not as rate
Relative Risk
RR= CI1/CI0
(A/A+B)/(C/C+D)
Interpretation:
RR= 1 there is no association (change to get sick is the same for exposed and unexposed)
RR>1 There is a positive association, the risk to get sick for de exposed is higher than the
unexposed.
RR<1 there is an inversive association, the risk to get sick is smaller for the exposed group
then for the unexposed group.
The change to get sick is .. times higher for the exposed people compared to the unexposed.
Incidence Density Ratio
RR= = ID1/ID0= IDR
Attributive risk
AR= CI1-CI0
(A/A+B) – (C/C+D)
Shows the risk difference between exposed people and unexposed people.
Association Measures
INCIDENCE
- The number of new cases in a given period in a specified population (time must be
specified)
- Measures the rapidity with which new cases are occurring in a population
- can be expressed:
o in absolute numbers
o in terms of cumulated incidence
o in terms of incidence density
Formula to go from IR to Ci
CI= 1-e^-(i x t)=
Cumulative incidence:
N of new cases
CI = x 10n
population at risk at beginning
CI 0= A/A+B Unexposed people
CI 1 = C/C+B Exposed people
CIt = A+B/N= 1−exp(−∑ Iι Δtι)
Proportion of people that develop disease during a certain time interval
Interpretation:
The changes to get sick when exposed is ...% in .. years
, Cumulative Incidence Rate
CIR= RR
Disease D+ No disease D- Total
Exposed E+ A B a+b
Non Exposed E- C D c+d
Total E F N
Incidence rate/ Incidence Density
IR= ID
ID= exposed/ population x Time
Exposed/ person-time
N of new cases
ID= x 10 n
total person−time of observation
ID= number of Incidence Cases/person years at risk = M1/L
IDe+ ¿ D 1=a /L 1
IDe-= D0 = b/Lo
Describes trends in diseases and evaluates impact of primary prevention programmes
E+ E- TOT
D+ A B Mt
PT L1 L0 L
The number of NEW cases per unite of population per unit of time.
IDR = incidence density ratio= ID1/ID0
IDD= incidence density difference= ID1-ID0
PREVALENCE
Prevalence= number of existing cases in the pop at a certain time/ number of persons in the
population at that certain time.
( people with disease at a specified time)
P= x 10n
( population at risk at the specified time)
I x ∆ t x ( N−P)
Proportion of people who HAVE the disease on a certain moment or in a certain period of
time.
Point Prevalence:
- Measures the frequency of disease at a given point in time
- Applies when the data has been collected at one point in time
, - P=C / N
o C= number of observed cases at time t
o N= population size at time t
Period prevalence
- Measures the frequency of disease over some time
- Applies when the data has been collected over a period of time
- P=C + I /N
o C= number of prevalent cases at the beginning of the time period
o I= number of incidence cases that develop during the period
o N= population size for this same time period
The relation between Prevalence and Incidence:
- Prevalence depends on:
Incidence (I). duration of the disease (D)
- Change in prevalence from one time period to another may be the result of changes
in Incidence rate, changes in the duration of disease or both
Case Fatality
Place in relation of number of deaths from a disease to the number of cases
- Reflects severity
- Can be expressed as:
o Proportion
o Ratio
o Not as rate
Relative Risk
RR= CI1/CI0
(A/A+B)/(C/C+D)
Interpretation:
RR= 1 there is no association (change to get sick is the same for exposed and unexposed)
RR>1 There is a positive association, the risk to get sick for de exposed is higher than the
unexposed.
RR<1 there is an inversive association, the risk to get sick is smaller for the exposed group
then for the unexposed group.
The change to get sick is .. times higher for the exposed people compared to the unexposed.
Incidence Density Ratio
RR= = ID1/ID0= IDR
Attributive risk
AR= CI1-CI0
(A/A+B) – (C/C+D)
Shows the risk difference between exposed people and unexposed people.
