Identifying the Study Design
When reading a paper, it isn’t always clear what the
study design is. Sometimes there is a combination of
strategies. However, you should think about what
the predominant design features are.
• Provides a framework for thinking about the study.
• Alerts you to weaknesses in some study designs.
Identifying the Study Design
Is it based on information about individuals?
Or averages in populations?
Correlational
(Ecologic)
Identifying the Study Design
Is there just one group?
X XX XX X
XXX
8 people
with bird flu
Did all subjects have the disease? (Case Series)
Did they evaluate presence of disease and risk factors
at the same point in time?
(Cross-sectional Survey) Do you have heart disease?
Are you active?
Identifying the Study Design
Two or more groups being compared?
• How were they selected? Did they find people with
disease [cases] and then find a comparison group
without disease [controls]? (Case-Control)
X X XX
XXX X
Compare past exposures
• Identify non-diseased people & group them by risk
factor status? Then follow them longitudinally to
compare incidence? (Cohort Study)
X X
X
Compare incidence over time
X
In prospective cohort studies conception, design, &
enrollment occur before anyone develops the outcome.
Enroll non-diseased subjects;
collect baseline exposure data
Prospective
X
Obese
Follow up at intervals to get
accurate outcome data.
Lean
Compare incidence over time
Retrospective
Exposed
Not
Exposed
Identify a cohort retrospectively
(e.g. tire manufacturing workers
vs. desk employees. Look at what
subsequently happened to them.
Compare incidence over time
X
X
X
X
X
X
X
Identifying the Study Design
Did the investigators assign subjects to a treatment or
intervention and follow them to compare outcomes?
(Clinical Trial)
Aspirin
X
Compare incidence over time
Placebo
X
X
X
Oral Contraceptives & Liver Cancer. Previous case reports of
liver cancers in women on OCs. The authors contacted all cancer
registries & collected information on all females with liver tumors.
OC
Use
Yes
No
Unknown
Age Category
16 - 25
26 - 35
%
%
31
43
20
10
49
48
What kind of study was this?
1.
2.
3.
4.
5.
Case series
Case-control study
Retrospective cohort
Prospective cohort
Randomized clinical trial
0%
0%
0%
0%
0%
36 - 45
%
22
29
49
State
Annual per capita
Tobacco Sales
Lung Cancer Mortality Rate in
1965/100,000 pop.
Alabama
$600
92
Florida
$450
75
Georgia
$500
80
North Carolina
$550
66
Virginia
$400
45
Alaska
$200
35
Massachusetts
$150
33
New York
$175
20
New Jersey
Rhode Island
1.
2.
3.
4.
5.
What kind of$200
study is this?
Case series
Case-control
Retrospective cohort
Cross-sectional survey
Correlational (ecologic)
$250
0%
0%
0%
0%
0%
23
22
1830: Villerme notes that mortality
varies among districts in Paris. He
tried to correlate mortality with the
distance of the arrondissement from
the Seine River, the relationship of
the streets to the prevailing winds,
the arrondissement's source of water
and local climatological factors such
as soil type, exposure to the sun,
elevation and inclination of the
arrondissement.
Villerme found that mortality correlated closely with the degree
of poverty in the arrondissement (estimated as the % of people
exempted from tax). The findings did not spark action.
poorest
wealthiest
Villerme found that mortality correlated closely with the degree
of poverty in the arrondissement (estimated as the % of people
exempted from tax). The findings did not spark action.
poorest
What kind of
study was this?
1.
2.
3.
4.
5.
Case series
Case-control
Retrospective cohort
Cross-sectional survey
Correlational (ecologic)
wealthiest
20%
20%
20%
20%
20%
Median
Household
Income
Premature
Deaths
/100,000
Lynn
38000
470
Lowell
40000
466
Springfield
30000
459
Newton
89000
218
Brookline
68000
233
Barnstable
47000
275
500
450
400
350
300
250
200
150
100
50
0
0
20000
40000
60000
80000
100000
Investigators in Bergen, Norway sent questionnaires
about respiratory health, allergies, smoking habits,
and occupational respiratory exposures to a random
sample of residents between the ages of 15-70. After
two reminders, 2,819 responses were obtained. Of
these, 1,646 reported exposure to tobacco smoke
from other members of their immediate family.
What kind of study was this?
1.
2.
3.
4.
5.
Case series
Case-control
Retrospective cohort
Cross-sectional survey
Correlational (ecologic)
0%
0%
0%
0%
0%
A study in N. Engl. J. Med. examined whether eating a
Mediterranean diet had any association with mortality in Greek
adults. A baseline questionnaire was used to determine how
closely the subjects followed a traditional Mediterranean diet,
and the group was followed for 2 years, during which they
determined the cause of death among all subjects who died.
Mediterranean Diet Score
Deaths in 2 yrs
Alive
Total
High (close adherence)
44
2586
2,630
Medium
61
3747
3,808
Low (poor adherence)
74
2383
2,457
1.
2.
3.
4.
What kind of study was this?
0%
Case-control
Retrospective cohort
Prospective cohort
Randomized clinical trial
0%
0%
0%
Bacteremia, Fever, and Splenomegaly Caused by a Newly
Recognized Bartonella Species. Eremeeva, et al.: N Engl J Med
2007;356:2381-7. A 43-year-old American woman developed a fever after
traveling in Peru for 3 weeks. She visited Lima and Nazca and then traveled
to the Sacred Valley of Urubamba, followed by Cuzco and Machu Picchu,
where she hiked. She received numerous insect bites. Sixteen days after
returning to the US she developed fever, insomnia, muscle aches, nausea,
headache, and mild cough. At the hospital she was found to have anemia
and an enlarged spleen (splenomegaly). Laboratory tests determined that
her blood was infected with a genus of bacterium called Bartonella.
1.
2.
3.
4.
5.
6.
What kind of study?
0%
Case report
Case series
Case-control
Retrospective cohort
Clinical trial
Ecologic
0%
0%
0%
0%
0%
In 2003 a mass immunization against cholera was conducted in
Beira, Mozambique. The following year there was an outbreak of
El Tor Ogawa cholera in Beira. To assess the usefulness of the
vaccine investigators compared the frequency of vaccination
between persons with culture-confirmed cholera severe enough to
have prompted them to seek treatment and age- and sex-matched
neighborhood controls who did not have diarrhea.
Study type?
1.
2.
3.
4.
5.
6.
Case series
Cross-sectional
Case-control study
Retrospective cohort
Prospective cohort
Clinical trial
0%
0%
0%
0%
0%
0%
Risk of kidney failure associated with the use of
acetaminophen, aspirin, and nonsteroidal antiinflammatory
drugs. Perneger TV, et al. People who take analgesic drugs
frequently may be at increased risk of chronic kidney failure.
These authors used a kidney dialysis registry to find 716
patients with kidney failure; they randomly selected 361
subjects without kidney disease from the same geographic
area. They used phone interviews to estimate their cumulative
past use of analgesics and compared the two groups.
What kind of study?
1.
2.
3.
4.
5.
Case series
Case-control
Retrospective cohort
Prospective cohort
Clinical trial
0%
0%
0%
0%
0%
Adiposity as Compared with Physical Activity in Predicting
Mortality among Women. Hu et al.: N Engl J Med
2004;351:2694-703. In 1976 the Nurse’s Health Study
enrolled 121,700 female RNs who completed a mailed
questionnaire regarding their medical history & lifestyle. The
women have returned follow up information every two years.
This study grouped them by exercise level & BMI and
compared mortality rates among different levels of these two
risk factors.
Type of study?
1.
2.
3.
4.
5.
Cases series
Case-control
Retrospective cohort
Prospective cohort
Clinical trial
0%
0%
0%
0%
0%
Glucosamine, Chondroitin Sulfate, and the Two in Combination for
Painful Knee Osteoarthritis. Clegg, et al. N Engl J Med
2006;354:795-808.
Glucosamine and chondroitin sulfate are orally administered
substances that have been used for years to treat joint problems in
horses. Since they are relatively non-toxic there has been
increasing interest in them for treating osteoarthritis, but there is
controversy about their efficacy. These investigators randomly
assigned 1583 patients with symptomatic knee osteoarthritis to
receive 1500 mg of glucosamine daily, 1200 mg of chondroitin
sulfate daily, both glucosamine and chondroitin sulfate, 200 mg of
celecoxib daily, or placebo for 24 weeks. The primary outcome
measure was a 20 percent decrease in knee pain from baseline to
week 24. The primary outcome measure was whether the patient
achieved a 20 percent decrease in pain as measured by the
WOMAC pain subscale, a standardized, previously validity tool for
assessing joint pain.
What kind of study is this?
These investigators randomly assigned 1583 patients with
symptomatic knee osteoarthritis to receive 1500 mg of
glucosamine daily, 1200 mg of chondroitin sulfate daily, both
glucosamine and chondroitin sulfate, 200 mg of celecoxib
daily, or placebo for 24 weeks. They compared the groups
with respect to decrease in knee pain from baseline to week
24 using the WOMAC pain subscale, a standardized,
previously validity tool for assessing joint pain.
Type of study?
1.
2.
3.
4.
5.
Case series
Case-control
Retrospective cohort
Prospective cohort
Clinical trial
0%
0%
0%
0%
0%
The frequency of new cases of
tuberculosis in Boston during a
given calendar year is…
1.
2.
3.
4.
Prevalence
Cumulative incidence
Incidence rate
None of the above
0%
0%
0%
0%
The % of deceased males who are found
to have prostate cancer at autopsy?
1.
2.
3.
4.
Prevalence
Cumulative incidence
Incidence rate
None of the above
0%
0%
0%
0%
The % of people who have malaria that is
resistant to treatment with chloroquine?
1.
2.
3.
4.
Prevalence
Cumulative incidence
Incidence rate
None of the above
0%
0%
0%
0%
The number of newly diagnosed brain
tumors in a calendar year per 100,000
Massachusetts children?
1.
2.
3.
4.
Prevalence
Cumulative incidence
Incidence rate
None of the above
0%
0%
0%
0%
The number of previously healthy
women who had a stroke during
40,000 person-years of follow up?
1.
2.
3.
4.
Prevalence
Cumulative incidence
Incidence rate
None of the above
0%
0%
0%
0%
In General:
 Express prevalence and cumulative
incidence as % or # per 1,000 or 10,000, etc.
 Express incidence rate as # per 1,000
person-yrs or 10,000 person-years, etc.
 Remember to state the time period of
observation for cumulative incidence.
However, for the exercises that follow and for post class
quiz #2, you will need to answer with the specific
numerator and denominator for some questions, to make
sure that you are including and excluding the right subjects.
Study population of 1,000. Dashed line = disease present (Lung Cancer)
Patients 1, 2, 3, & 4) had the disease before the study began.
During the year of the study, 6 new cases occur (start of dashed lines).
Among the total of 10 cases, there were 6 deaths during the year
The 990 other individuals in the study did not become ill or die.
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
X
6
----------------------------Dead
X
7
X-------------------------------------------------------------------------Alive
8
X------------Dead
9
X------------------------------------------------------------Alive
10
X---------------------------------------Dead
Prevalence of disease on:
Jan. 1, 1994?
July 1, 1994?
Dec. 31, 1994?
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Prevalence on Jan. 1, 1994?
A.
B.
C.
D.
E.
F.
0/1000
4/1000
6/1000
4/996
5/996
6/996
0%
0%
0%
0%
0%
0%
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Prevalence on July 1, 1994?
A.
B.
C.
D.
E.
F.
4/997
4/1000
6/1000
4/996
5/996
5/997
0%
0%
0%
0%
0%
0%
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Prevalence on Dec. 31, 1994?
A.
B.
C.
D.
E.
F.
4/1000
4/996
4/994
6/1000
6/996
6/994
0%
0%
0%
0%
0%
0%
Study population of 1,000. Dashed line = disease present (Lung Cancer)
Patients 1, 2, 3, & 4) had the disease before the study began.
During the year of the study, 6 new cases occur (start of dashed lines).
Among the total of 10 cases, there were 6 deaths during the year
The 990 other individuals in the study did not become ill or die.
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
X
6
----------------------------Dead
X
7
X-------------------------------------------------------------------------Alive
8
X------------Dead
9
X------------------------------------------------------------Alive
10
X---------------------------------------Dead
What was the cumulative incidence during 1994?
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Cumulative incidence in 1994?
A.
B.
C.
D.
E.
F.
4/1000
6/1000
10/1000
4/996
6/996
10/996
0%
0%
0%
0%
0%
0%
Study population of 1,000. Dashed line = disease present (Lung Cancer)
Patients 1, 2, 3, & 4) had the disease before the study began.
During the year of the study, 6 new cases occur (start of dashed lines).
Among the total of 10 cases, there were 6 deaths during the year
The 990 other individuals in the study did not become ill or die.
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
X
6
----------------------------Dead
X
7
X-------------------------------------------------------------------------Alive
8
X------------Dead
9
X------------------------------------------------------------Alive
10
X---------------------------------------Dead
What was the population mortality rate during 1994?
(Use population at beginning of year for calculation)
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Population mortality rate in 1994?
A.
B.
C.
D.
E.
F.
4/10 or 40%
6/10 or 60%
4/1000 or 0.4%
6/1000 or 0.6%
6/996
10/1000
0%
0%
0%
0%
0%
0%
Study population of 1,000. Dashed line = disease present (Lung Cancer)
Patients 1, 2, 3, & 4) had the disease before the study began.
During the year of the study, 6 new cases occur (start of dashed lines).
Among the total of 10 cases, there were 6 deaths during the year
The 990 other individuals in the study did not become ill or die.
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
X
6
----------------------------Dead
X
7
X-------------------------------------------------------------------------Alive
8
X------------Dead
9
X------------------------------------------------------------Alive
10
X---------------------------------------Dead
What was the case-fatality rate during 1994?
1994
Jan Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
1995
Dec Jan
1<---------------------------------------------------------------------------------------------------------Alive
2<---------------------------------------Dead
3<--------------------------------------------------------------Dead
4<-----Dead
5
-----------------------------------------------------------------------------------------------------Alive
6
----------------------------Dead
7
-------------------------------------------------------------------------Alive
8
------------Dead
9
------------------------------------------------------------Alive
10
---------------------------------------Dead
Case fatality rate?
A.
B.
C.
D.
E.
6/10 or 60%
6/1000 or 6%
6/996
4/996
4/10
0%
0%
0%
0%
0%
A new disease occurs with constant IR= 3/1000 person-yr.
Estimate the cumulative incidence over 5 years.
1.
2.
3.
4.
0.03 over 5 yrs.
0.010 over 5 yrs.
0.015 over 5 yrs.
0.030 over 5 yrs.
0%
0%
0%
0%
You should be able to calculate these measures
of disease frequency and measures of
association using a simple hand calculator.
Epi_Tools.XLS will also do them, but you need to
be able to do them without Epi_Tools for the
exams.
What does one measure and
compare in a case-control study?
1.
2.
3.
4.
5.
0%
Cumulative incidence
0%
Incidence rate
0%
Risk of disease
Frequency of past exposures
0%
Risk difference
0%
In a cohort study one may measure the degree of
association between an exposure and an outcome by
calculating either a relative risk or an odds ratio?
1.
2.
3.
0%
True
0%
False
I’m not sure 0%
In a case-control study one may measure the degree of
association between an exposure and an outcome by
calculating either a relative risk or an odds ratio.
1.
2.
3.
0%
True
0%
False
I don’t know.
0%
When is an odds ratio a legitimate
estimate of relative risk?
1.
2.
3.
4.
0%
Whenever one is conducting
a case-control study.
When the exposure is
0%
relatively uncommon.
When the outcome is
relatively uncommon.
0%
When the sample size is
large.
0%