Researchers can quantify the occurrence of disease by making use of measures of disease
frequency (MODF), such as:
Prevalence (and variants of this MODF: point, period, and lifetime prevalence)
Incidence (and variants of this MODF: cumulative incidence and incidence density)
Mortality
Case fatality rate (CFR)
1a. Define all these MODF. How should one calculate these measures?
Prevalence = the number of existing disease cases in the population
- Point prevalence: part of the population that is diseases at a certain point in time.
Number of disease cases
x 100 %
population
- Period prevalence: part of the population that had the disease in a certain period of
time. Use the mid-term population
number of disease cases within a period
( ( Nstart+ Nend)
2
)
x 100 %
- Life-time prevalence: part of the population that had the disease during their lifetime
Incidence = new cases of the disease in the population
- Cumulative incidence (CI) closed population
number of new cases period
CI = x 100 %
total number population at risk at T 0
- Incidence density rate (ID) open/dynamic and closed population
number of new cases period
ID= '
persontime at risk ( ' persontim e )
Mortality rate = total numbers of death within a period of time. The ‘incidence of death’,
calculating the prevalence of death isn’t really possible
number of deaths
Mortality rate= x 100 %
population at risk
, SEMINARS – GZW3024
Case fatality rate (CFR) = the number of deaths from a specified disease within a certain
period of time. The part of all the people with that disease that died.
CFR=number of deaths due ¿ the disease ¿ x 100 %
the number of individuals diagnosed withthe disease
1b. In which of the following designs can these MODF be calculated?
Case series
Ecological study
Cross-sectional study/survey
Case-control study
Prospective cohort study
Retrospective (or ‘historic’) cohort study
Randomized controlled trial (RCT)
Indicate in the table below (‘yes’/’no’) whether the specific MODF can be calculated in the
listed study designs:
Prevalence Incidence Mortalit CFR
y
Point Period Lifetime CI ID
Case series No No No No No No Yes
Ecological Yes Yes Yes Yes Yes Yes Yes
study
Cross- Yes Yes Yes No No No No
sectional
study
Case-control No No No No No No No
study
Prospective Yes Yes Yes Yes Yes Yes Yes
cohort
Retrospective Yes Yes Yes Yes Yes Yes Yes
cohort
RCT Yes Yes Yes Yes Yes Yes Yes
Homework assignment 2
The following table represents the results from a follow-up study over a period of 12
months. In this study the participants are continuously monitored for disease D during their
follow-up. It is assumed that all participants are at-risk as long as they are present in the
, SEMINARS – GZW3024
study population (indicated in yellow), except when they have the disease (indicated in
blue).
2a. What type of population – ‘open’ (or dynamic) or ‘closed’ (or cohort) – is under study?
This is an open/dynamic population, because not all participants are always present in the
study population and they don’t all enter the population at the start of the study.
2b. What are the sex-specific point prevalence’s of disease D on the second day of the 6th
month?
, SEMINARS – GZW3024
Point prevalence males = (2/17) x 100% = 11,8%
Point prevalence females = (2/24) x 100% = 8,3%
2c. Calculate the point prevalence of disease D for the total population on the second day
of the 6th month.
Point prevalence total population = (4/41) x 100% = 9,8%
2d. Suppose that people could not be cured from disease D. What would then have been
the point prevalence of disease D on the second day of the 6th month?
Point prevalence = (7/41) x 100% 17,1%
What are now the sex-specific point prevalence’s of disease D on the second day of the 6th
month?
Point prevalence male = (4/17) x 100% = 23,5%
Point prevalence female = (3/24) x 100% = 12,5%
2e. Calculate the incidence of disease D in the total population.
Because it’s an open population, you have to calculate the incidence density.
number of new cases period
'
persons x time at risk ( ' person tim e )
ID total population = (8+7)/ (178+187) = 15/365= 0,0410
Total time that people are in the population is 365 months; count the number of
months that every participant is part of the population (part of the yellow population
– blue population)
If someone gets sick two times, this participant counts as two cases
2f. Are males or females more likely to develop disease D? (Males are ID-numbers A1-A24,
females are ID-numbers B1-B26)
Males = (8/178) = 0,0449
Females = (7/187) = 0,0374
Males are more likely to develop the disease
2g. Suppose that due to improvements in the treatment of disease D, the disease duration
is reduced to 50%. What happens to incidence calculated in 2e and 2f?
The incidence density will be reduced, because you have more months ‘at risk’ (person time)
and less month’s sick. The number of new cases stays the same.
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