Exam preparation
Case 1
1. What is the difference between a retrospective cohort study and a case-control study?
When would you apply what?
In a retrospective cohort study the risks of developing a disease is determined and compared
to known exposure factors. The outcome measure is relative risk, as cumulative incidence
numbers can be calculated. Because a retrospective cohort study is longitudinal, exposure is
identified before the outcome, it is possible to assess causality (but not prove it). I would use
this type of study if I want to investigate the effect of a certain dietary compound on multiple
outcomes. I think it is not suitable to use this study design for rare diseases, as a very large
sample size would be needed then.
In a case control study, on the other hand, exposure factors are determined after a known
disease incidence. In other words, patients with a certain disease are selected and matched
with controls who come from the same population as the patients, then both cases and
controls are interviewed/questionnaire to find out anything that might be a risk factor for the
disease. The outcome measure is odds ratio. Case-control studies cannot provide any
information about the incidence or prevalence of a disease because no measurements are
made in a population based sample. Case-control studies may prove an association but they
do not demonstrate causation. I would use this study if you want to investigate the potential
risky exposure factors that can contribute to the development of relatively rare and unknown
diseases.
2. Why is evidence of prospective cohort studies considered to be weaker than that from
randomised controlled trials?
As the name already tells, randomised controlled trials can be highly controlled by a
researcher. In prospective cohort studies, as people just live their lives and the researches
observes events but does not control them. Besides this, in a RCT participants (which are as
much the same as possible) are randomly allocated to either the treatment or control group.
In cohort studies participants are naturally divided into groups based on their exposure
status, and this brings the problem of confounding. Namely, it could be that the participants
that were naturally divided in the exposure group are also relatively more exposed to other
factors that can affect the outcome measurements. Due to the higher risk for confounding on
cohort studies, the evidence is considered to be weaker than evidence from RCT.
3. What kind of study design would you propose to investigate the potential preventive effect
of whole grain products on T2D?
I think it is not ethical to do a long-term RCT in healthy people, as you cannot force people to
avoid whole grain food for a few years which makes them develop a chronic disease. Besides
this, compliance for such a long time is also very difficult. However, a short-term RCT for
individuals at risk for T2D (prediabetes) which determines the effect of whole grain food
consumption on outcome measures for T2D could be ethical. Especially if the control group is
instructed to maintain their habitual diet.
, To asses the long term affect of whole grain food consumption I think it would be better to
do a prospective cohort study which follows people that regularly consume whole grain
products and a group of people that do not. Although it cannot prove causality it can
certainly give an estimation of causality.
4. The following piece of text is extracted from the introduction section of the paper:
“Exposures to contaminated fluids from percutaneous needlesticks and laceration injuries
are serious hazards associated with postgraduate medical training. These injuries may result
in the transmission of blood-borne pathogens, including hepatitis and human
immunodeficiency viruses, and thus have significant occupational health implications.
Factors contributing to the occurrence of these percutaneous injuries in physicians have not
been well studied. We hypothesized that sleep deprivation may play a role in these
incidents.”
The authors decided to study the association between extended work duration and
rates of percutaneous injuries by means of a prospective cohort study.
Why is the choice of a prospective cohort study a reasonable one compared to an
experimental study or a case-control study?
First of all, an experimental study is not ethical. You cannot force people to extent work
durations with the risk of more injuries. A case-control design could be an option, but the
evidence of case-control studies is weaker than that from cohort studies. This has to do with
the high risk of confounding and bias in case-control studies. In case-control studies people
are asked to recall if they extended their work duration for an x period of time in the past,
this can be very hard and if people incorrectly remember this can lead to information bias.
This is not true for prospective cohort studies as people are followed over time and do not
have to recall if they extend work or not.
5. In the late 1970s, the hypothesis that environmental events (i.e. the death of a close relative,
change in residence, or change in work) increases the risk of sudden cardiac death in women
was postulated. This hypothesis was investigated in a case-control study conducted by
Cottington and colleagues.
Do you think that a case-control was justified to study this particular hypothesis? Explain
your answer.
Yes, I think that a case-control design is justified in this situation. As sudden cardiac deaths
among middle aged women are relatively rare, you need a very large sample size if you
would use a cohort study, which is expensive. Also people mostly easily recall deaths of close
relatives, change in residence and work as this are large events. However, I do not rule out
the option of a prospective cohort study.
6. Anorexia nervosa (AN) is a psychiatric disease that is associated with increased morbidity and
mortality. Multiple factors –environmental and biological– seem to play a role in the etiology
of this disease. In order to gain insight in the potential risk factors of AN, a study was
conducted among 50 women with AN according to the DSM-IV criteria (the ‘cases’) and 50
women without AN (the ‘controls’). The researchers employed several strategies to sample
the controls, including posters and advertisements in newspapers and other media. The
women with AN were recruited via a clinic for eating disorders.
Case 1
1. What is the difference between a retrospective cohort study and a case-control study?
When would you apply what?
In a retrospective cohort study the risks of developing a disease is determined and compared
to known exposure factors. The outcome measure is relative risk, as cumulative incidence
numbers can be calculated. Because a retrospective cohort study is longitudinal, exposure is
identified before the outcome, it is possible to assess causality (but not prove it). I would use
this type of study if I want to investigate the effect of a certain dietary compound on multiple
outcomes. I think it is not suitable to use this study design for rare diseases, as a very large
sample size would be needed then.
In a case control study, on the other hand, exposure factors are determined after a known
disease incidence. In other words, patients with a certain disease are selected and matched
with controls who come from the same population as the patients, then both cases and
controls are interviewed/questionnaire to find out anything that might be a risk factor for the
disease. The outcome measure is odds ratio. Case-control studies cannot provide any
information about the incidence or prevalence of a disease because no measurements are
made in a population based sample. Case-control studies may prove an association but they
do not demonstrate causation. I would use this study if you want to investigate the potential
risky exposure factors that can contribute to the development of relatively rare and unknown
diseases.
2. Why is evidence of prospective cohort studies considered to be weaker than that from
randomised controlled trials?
As the name already tells, randomised controlled trials can be highly controlled by a
researcher. In prospective cohort studies, as people just live their lives and the researches
observes events but does not control them. Besides this, in a RCT participants (which are as
much the same as possible) are randomly allocated to either the treatment or control group.
In cohort studies participants are naturally divided into groups based on their exposure
status, and this brings the problem of confounding. Namely, it could be that the participants
that were naturally divided in the exposure group are also relatively more exposed to other
factors that can affect the outcome measurements. Due to the higher risk for confounding on
cohort studies, the evidence is considered to be weaker than evidence from RCT.
3. What kind of study design would you propose to investigate the potential preventive effect
of whole grain products on T2D?
I think it is not ethical to do a long-term RCT in healthy people, as you cannot force people to
avoid whole grain food for a few years which makes them develop a chronic disease. Besides
this, compliance for such a long time is also very difficult. However, a short-term RCT for
individuals at risk for T2D (prediabetes) which determines the effect of whole grain food
consumption on outcome measures for T2D could be ethical. Especially if the control group is
instructed to maintain their habitual diet.
, To asses the long term affect of whole grain food consumption I think it would be better to
do a prospective cohort study which follows people that regularly consume whole grain
products and a group of people that do not. Although it cannot prove causality it can
certainly give an estimation of causality.
4. The following piece of text is extracted from the introduction section of the paper:
“Exposures to contaminated fluids from percutaneous needlesticks and laceration injuries
are serious hazards associated with postgraduate medical training. These injuries may result
in the transmission of blood-borne pathogens, including hepatitis and human
immunodeficiency viruses, and thus have significant occupational health implications.
Factors contributing to the occurrence of these percutaneous injuries in physicians have not
been well studied. We hypothesized that sleep deprivation may play a role in these
incidents.”
The authors decided to study the association between extended work duration and
rates of percutaneous injuries by means of a prospective cohort study.
Why is the choice of a prospective cohort study a reasonable one compared to an
experimental study or a case-control study?
First of all, an experimental study is not ethical. You cannot force people to extent work
durations with the risk of more injuries. A case-control design could be an option, but the
evidence of case-control studies is weaker than that from cohort studies. This has to do with
the high risk of confounding and bias in case-control studies. In case-control studies people
are asked to recall if they extended their work duration for an x period of time in the past,
this can be very hard and if people incorrectly remember this can lead to information bias.
This is not true for prospective cohort studies as people are followed over time and do not
have to recall if they extend work or not.
5. In the late 1970s, the hypothesis that environmental events (i.e. the death of a close relative,
change in residence, or change in work) increases the risk of sudden cardiac death in women
was postulated. This hypothesis was investigated in a case-control study conducted by
Cottington and colleagues.
Do you think that a case-control was justified to study this particular hypothesis? Explain
your answer.
Yes, I think that a case-control design is justified in this situation. As sudden cardiac deaths
among middle aged women are relatively rare, you need a very large sample size if you
would use a cohort study, which is expensive. Also people mostly easily recall deaths of close
relatives, change in residence and work as this are large events. However, I do not rule out
the option of a prospective cohort study.
6. Anorexia nervosa (AN) is a psychiatric disease that is associated with increased morbidity and
mortality. Multiple factors –environmental and biological– seem to play a role in the etiology
of this disease. In order to gain insight in the potential risk factors of AN, a study was
conducted among 50 women with AN according to the DSM-IV criteria (the ‘cases’) and 50
women without AN (the ‘controls’). The researchers employed several strategies to sample
the controls, including posters and advertisements in newspapers and other media. The
women with AN were recruited via a clinic for eating disorders.
