-: Hello and welcome to module two
of Environmental Public Health.
Environmental and Occupational Epidemiology draws
on the same tools as epidemiology generally.
I understand that some
of you have already taken epidemiology one, but for those
of you who haven't, some of this material may be new.
If you've already taken it, then this might be
a good time to revisit some of the basics.
There are a few keywords in this definition
of epidemiology that I wanna go through really quickly.
First, let's consider the distribution
of the disease in a population.
Distribution refers to who has the disease,
which age groups, which sex and socioeconomic status.
Where are cases of the disease geographically located?
When did the cases of disease occur?
These questions establish persons, place and time
and make up what is known as descriptive epidemiology.
Next, let's consider determinants of disease.
Smoking is a determinant for lung cancer
and sun exposure and sunburns are determinants for melanoma.
These are often referred to
as risk factors or causal factors.
There are several different kinds
of epidemiological studies.
Descriptive studies describe the disease
with regards to persons, place and time
but they don't formally test any hypotheses
about exposure and disease relationships.
Ecological studies
which are sometimes called correlational studies
examine exposure-disease relationships
at a population level.
For example, a cancer epidemiologist might note
that there's a higher rate of leukemia
in the south of a city near a hazardous waste site.
Does this mean
that the hazardous waste site is causing leukemia?
You really can't say because you don't know
for certain whether the people
with the risk factor are the same as those with the disease.
You would need to do additional studies to
assess this relationship.
Ecological studies are particularly useful
for hypothesis generation though.
Etiological or analytical studies test a specific exposure
disease hypothesis among individuals.
They're divided into experimental and observational studies.
Randomized control trials fall
under the experimental study category
and these are the gold standard for evidence
and are frequently used in clinical medicine to determine
if one treatment outperforms another.
However, they're rarely used in environmental epidemiology
because it isn't ethical to randomize people
to environmental hazards.
The exception to this is randomizing people
to intervention trials in which a proposed method
of reducing exposure or a protective factor is assessed.
Observation studies are the most frequently used
in environmental epidemiology, but they're subject
to some confounding, and that's an inherent limitation.
I'll walk through different types
of observational studies in the next few slides.
So first I wanna talk about cohort studies.
Cohort studies are useful when separate groups
of exposed and non-exposed individuals can be assembled.
Some studies follow large cohorts
of people over time to assess a variety
of exposure-disease relationships.
These are prospective studies
in which the groups are assembled
and then followed forward in time.
These are typically expensive
and take a long time to conduct
because epidemiologists have to wait
for the disease events to occur.
However, they can provide more accurate information
on exposure, especially if you're taking measurements
at the start of the study.
Cohort studies can also be done retrospectively.
In that case, epidemiologists would assemble groups
of people with a past exposure
and assess their current health status.
These types of studies are usually less expensive
and don't take as long to conduct
but sometimes some pieces of information may be missing
like the exact exposure level.
Cohort studies as a rule of thumb
can be best for when the exposure is rare
but the outcome or the disease is common.
In contrast to cohort studies
case-control studies tend to be good
for when the exposure is common, but the disease is rare.
So in a case-control study, epidemiologists select cases
with a disease and cases without that disease
and look back in time to assess prior exposure.
Exposures are then compared between the two groups
so between the cases and the controls to determine
if a particular exposure is more common
in those with the disease than those without.
This results in an odds ratio.
If the case control study is well designed
it should give the same answer
as if it were conducted as a cohort study.
But it can be really difficult to design a case
control study very well.
Controls may not be representative
of the general population.
It may be difficult to determine past exposures
with certainty
and cases may remember past exposures differently
than your control group does.
This is referred to as recall bias.
Cross-sectional studies measure exposure
and disease at the same time.
Since temporality can usually not be inferred
this is considered a weaker study design
than cohort or case-control studies.
However, cross-sectional studies are often
the only possible design for surveys.
Findings from cross-sectional studies can then
often indicate when a more rigorous study is warranted.
So here is a graphical depiction
of the different types of epidemiological studies.
There are a number of different ways that you
could depict how the studies are similar or different
but I like this version and it might be helpful
to those of you who are visual learners.
This slide splits studies into observational studies
and experimental studies.
The observational studies are then split further
into population-based studies and individual-based studies.
The individual-based studies are typically
stronger study designs.
The individual based studies are split
into descriptive and analytical studies.
The descriptive studies have no comparison group
and include things like case reports or case series.
The analytical studies must have a comparison group
and include the study design types that we just discussed.
So you have cohort studies where groups are assembled based
on exposure and then followed
over time to determine whether those who are exposed
and those who weren't exposed end
up with the disease outcome.
Case-control studies where cases have the outcome
of interest and controls don't, and we look back
in time to see if their exposures differ
and then cross-sectional studies that look
at exposure and outcome at the same time.
On the other side of the slide,
under experimental studies, we have randomized studies
including randomized control trials which are often referred
to as clinical trials and as well as non-randomized studies.
This includes quasi-experimental, field trials
or community trials
and often we can't truly randomize people
into two groups based on the practicalities of the study.
So now that we've walked
through the types of epidemiological studies
let's do a quick self-check to see if we have
fully understood the differences between these studies.
This is the first self-check.
Feel free to pause the video
as you think about your answers for these questions.
The first self-check reads:
"A study in which children are randomly assigned to receive
either a newly formulated vaccine
or the currently available vaccine, and then
are followed to monitor for side effects and effectiveness
of each vaccine is an example of which type of study?
Select all the answers that apply."
So now I'm gonna show you the answers.
A. experimental and E. clinical trial.
So a study in which subjects are randomized
into two intervention groups and then monitored to
identify health outcomes is a clinical trial
which is a type of experimental study.
It's not a cohort study
because that term is limited to observational studies.
Okay, let's try another one.
"The Iowa Women's Health study
in which researchers enrolled 41,837
women in 1986 and collected exposure and lifestyle
information to assess the relationship
between these factors and the subsequent occurrence
of cancer is an example of which types of study?
Select all the answers that apply."
Okay, I'm gonna show you the answers now.
So pause it if you aren't ready.
So B. observational and C. cohort.
So a study that assesses
but doesn't dictate exposure and follows
up with the participants to document their
subsequent occurrence of disease is an observational
cohort study.
Remember in the last example where exposure was dictated
because children were randomized
into two separate groups to receive one vaccine or another.
That would be an experimental
but because this study assesses exposure
and it doesn't dictate exposure
that makes it an observational study.
Alright, let's try another one.
"British investigators conducted a study to compare measles
mumps, rubella or MMR vaccine history
among 1,294 children
with pervasive development disorder."
So that would include things like autism
or what was once referred to as Asperger syndrome.
"and 4,469 children
without such disorders.
"They found no association," just to be clear.
But "this is an example
of which types of study select all answers that apply."
Okay, I'm gonna show you the answers now.
B. observational and D. case-control.
So this is a study in which the subjects were enrolled
on the basis of having or not having the health outcome.
So that makes it an observational case-control study.
Remember, for case control studies, people are split
up based on having the outcome, whereas in a cohort study
the groups are split based on exposure.
Alright, let's try another one.
"A cohort study differs from a case-control study
in that" A. subjects are enrolled or categorized
on the basis of their exposure status in a cohort study
but not in a case-control study.
B. subjects are about their exposure status
in a cohort study, but not in a case-control study.
C. cohort studies require many years to conduct
but case-control studies do not,
or D. cohort studies are conducted to investigate
chronic diseases.
Case-control studies are used for infectious diseases.
Here are the answers.
A. So the key difference between a cohort
and a case-control study is that in a cohort study
subjects are enrolled on the basis of their exposure
whereas in a case-control study
subjects are enrolled on the basis of
whether they have the disease of interest or not.
So we just covered that in the last slide as well
so hopefully we've really hammered it home.
Both types of studies assess exposure and disease status.
While some cohort studies have been conducted
over several years, others
especially when it's related to an outbreak
can be conducted really rapidly even in a matter of days.
Either type of study can be used to study all sorts
of different health problems, including both infectious
and chronic diseases or non-communicable diseases.
Okay, here's the final self-check question.
"A key feature of a cross-sectional study is that:"
A. it usually provides information
on prevalence rather than incidents.
B. it is limited to health exposures
and behaviors rather than health outcomes.
C. it is more useful for descriptive epidemiology
than it is for analytic epidemiology
and D. it is synonymous with survey.
So here are the answers.
A, C and D all hold true.
So a cross-sectional study or a survey provides a snapshot
of the health of a population in one slice of time
so it assesses prevalence rather than incidents.
As a result, it's not as useful as a cohort study
or a case-control study for analytic epidemiology
but a cross-sectional study can still easily
measure the prevalence of exposures and outcomes.
So hopefully this has helped you get up
to speed on the different types of epidemiological studies.
Feel free to go back through these questions
and test yourself again if you find it helpful.