Samenvatting GZW3024 Alle seminars (1-27)!

GZW3024: Seminars (1-27)
Seminar 1: Measures of disease frequency and study design
Assignment 1
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
population )
x 100 %


- 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
x 100 %
( N start + N end )
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- 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.
CI = (Number ofnew cases∈ period P
Total number poplation ❑' a t ris k ' at T 0 )
x 100 %


- Incidence density rate (ID)  open/dynamic and closed population.
ID=
(
number of new cases∈ period P
persons x time ❑' a t ris k ' (' person tim e ' ) )
Mortality rate = total numbers of death within a period of time. De ‘incidence of death’,
calculating the prevalence of death isn’t really possible.

number of deaths
Mortality rate= x 1000
population ❑' a t risk '




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, 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=t h e number of deat h s duo ¿ thedisease ¿ x 100 %
t h e number of individuals diagnosed wit h t h e disease

1b. In which of the following designs can these MODF be calculated?
 Case series;
 Ecological study;
 Crossectional 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
Mortality CFR
Lifetime
Point- Period- CI ID
-

Case series No No No No No No Yes

Ecological
Yes Yes Yes Yes Yes Yes Yes
study

Crosssectional
Yes Yes Yes No No No No
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




Assignment 2
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 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?
(Justify your answer.)
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 prevalences of disease D on the second day of the 6th
month? (Males are ID-numbers A1-A24, females are ID-numbers B1-B26)
Point prevalence males = (2/17) x 100% = 11,8%
Point prevalence females = (2/24) x 100% = 8,3%


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, 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 prevalences 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.

ID total population = (8+7) / (178 + 187) = = 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.
- If someone gets sick two times, he 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
So, 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 incidences calculated in 2e and 2f?
The incidence density will be reduced, because you have more months ‘at risk’ (person
time) and less months sick. The number of new cases stays the same.

Assignment 3
The following table shows a series of cumulative incidences of disease D (per 100,000
per year) by age and calendar time in a follow-up study in which birth cohorts (i.e.
individuals that were born in the same year(s), born in the same age group) are
measured multiple times.




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