2: Measures of Frequency

Question 1

Definition of Health, Disease and Case

Answer 1

Health
state of comlete physical, mental and social well being and not merely the absense of disease

Disease
opposite of health, confirmed by diagnosis, definitions subject to agreement and change over time

Case
a person who has the diasease of interest, defined by inclusion and exclusion criteria
- uniform and standarized measurement and documentation

Question 2

Population
- definition
- fixed vs open
- total vs at risk

Answer 2

Definition
group of persons with well defined common characteristics (inclusion / exclusion criteria)

• e.g. demographics, disease related, geographics, group membership, time

Fixed
Membership based on event, therefore fixed, will never gain new members, e.g. People that studied in Lübeck in 2020

Open
Membership based on a condition, membership is transitory, e.g. inhabitants of Lübeck

at risk
must be possible to move from denominator to numerator

case / population at risk

e.g. immune people are not a risk, but still member of total population

Question 3

What is attack rate, when is it used?

Answer 3

analysis of outbreaks where most cases occur within hours or days, cases occuring later are not counted

AR = exposed and ill / exposed

time must be stated explicitly

Question 4

Incidence vs prevalence

Answer 4

Prevalence
exsisiting cases, proportion of poulation that has disease

Incidence
occurence of new cases, how quickly people catch the disease

Question 5

Cummulative incidence

Answer 5

• incidence proportion

new cases in defined time period /
poulation at risk at start of time period

• fixed population

Question 6

Incidence density rate

Answer 6

new cases in defined time period / person time at risk

Question 7

Point prevalence

Answer 7

people with disease at given time point / populatio size at given time point

Question 8

Period prevalence

Answer 8

People with disease in time period /
average population size in time period

Question 9

Comparison of incidence and prevalence measures

• When is cummaltive incidence equal to incidence density
• When is point prevalence equal to period prevalence ?
• When is incidence density equal to period prevalence ?
• Why cant we look at time point for incidence ?

Answer 9

When is cummaltive incidence equal to incidence density?

When poulation is fixed and disease is over in an instant, because then population time at risk = poulation at risk

When is point prevalence equal to period prevalence ?
When period is so short it can be treated as time point

When is incidence density equal to period prevalence ?
When the disease is over in an instant, because then no cases with disease start prior to investigated period are prevalent and the person time at risk is equal to population at risk in the period

Why cant we look at time point for incidence ?
A new case happening at a time point is near 0

Question 10

Mortality for fixed and open poulation

Answer 10

Fixed
cummulative mortality

Open or fixed
Mortality density

• same formula as incidence

Question 11

Difference between mortality and lethality

Answer 11

Mortality
Deaths per populations

Letahlity
Case fatality rate: Deaths per patient

• therefore lethality is higher