Homework Assignment #1
DEFINITIONS AND LEVELS OF PREVENTION
General Directions for ALL Homeworks:
(1) Write answers neatly on a separate sheet of paper
(2) Bring your answers to Lab to use for discussion.
(3) We recommend you not write your answers on your homework question sheet.
You may find it helpful to use the homework questions as review for the
midterm and final exams and it is easier to test your knowledge if the answers
are not shown on the question page.
(4) HOMEWORK IS NOT GRADED. However, we provide an idea of the number
of points that would be given, so you know how much is required for each
question
A. DEFINITIONS and CONCEPTS: Give a brief definition, and an example if
appropriate. You may use information from the lecture, the textbooks, or you may
use other resources, such as the Glossary in Section K of Part III of the Syllabus.
1.
What is the definition of epidemiology? (10 points)
2. Describe briefly how epidemiology is similar to clinical medicine and how it
differs. (2 or 3 sentences) (10 points)
3. Describe briefly how a study population differs from a source population and
how a source population differs from a target population. (20 points)
4. Describe the major difference between an observational study
and an experimental study. If you wanted to know if frequently eating
McDonalds Big Macs causes people to gain weight, describe (in 1 or 2
sentences) how you would answer this with an observational design versus an
experimental design? Which approach do you think is better? Why? (30 points)
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B. LEVELS OF PREVENTION: Assume the activities listed below are being
performed or requested by a primary care physician. Write the level of prevention
(primary, secondary, tertiary or not a preventive activity) that each activity
represents and, very briefly, why you think so? (Note: Use format as shown
below.) (2 points each).
LEVELS: Primary
Secondary
Tertiary
None – diagnosis or treatment, not a preventive activity
Example: Advising a young adult to stop smoking
Model Answer: Primary prevention because the activity is aimed at reducing a
risk factor for disease.
1.
Prostate specific antigen (PSA) test for a 65 year old man during his annual
physical exam.
2.
Use of earplugs by employees in an automobile plant with loud, assembly-line
equipment.
3.
Ordering a fecal occult blood test for an asymptomatic 60-year old man.
4.
Prescribing hormone replacement therapy for a woman with a family history of
osteoporosis.
5.
Measles, mumps and rubella vaccination for a 12 month old child.
6.
HIV test for a healthy, pregnant woman at one of her prenatal visits.
7.
Stress reduction class for office employees with high blood pressure.
8.
Nutritional counseling for elementary school children.
9.
Allergy skin testing and desensitization shots for an asthmatic 10-year old.
10.
Speech and occupational therapy for a 60 year old following a stroke.
11.
Prescribing a cholesterol-lowering drug to a man recovering from a heart attack.
12.
Conducting an annual skin test for TB in a medical student.
13.
Conducting a skin test for TB in a nonsmoker presenting with a chronic cough.
14.
Recommending that a computer programmer with an injured back use
an ergonometrically adjustable chair and computer station.
15.
Mammogram for 45 year old woman with breast lump discovered during self
breast exam.
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Homework Assignment #1
DEFINITIONS AND LEVELS OF PREVENTION
ANSWERS
A.
DEFINITIONS and CONCEPTS
1. Epidemiology is the study of the distribution and determinants of health-related
states in the human population.
2. Epidemiology and clinical medicine are both aimed at studying diseases and other
health-related states among humans. However, epidemiology involves studying
diseases at the population level, while clinical medicine involves treating disease in
individual patients.
3. A study population represents the group of people that are selected from a larger
source population to be included in a research study and on whom we collect data..
A target population represents the people that we would like to make causal
inference, while a source population represents are those people who are eligible to
be in the study because they meet criteria determined by the scientists; they are the
group of people that may actually be selected for inclusion in a study.
4. The major difference is that investigators only observe subjects exposures in an
observational study, while investigators assign exposures to subjects in an
experimental study.
To investigate whether eating McDonalds Big Mac leads to weight gain, an
observational design could involve following a group of people over time, some who
eat Big Macs frequently and some who do not, to determine whether the people who
eat Big Macs are more likely to gain weight than people who do not eat Big Macs.
An experimental design could involve assigning people to one of two groups: the first
group would be instructed to eat a Big Mac a specified number of times per week
and the second group would be instructed to eat another healthier type of sandwich.
At the end of the study period it could be determined whether there was a difference
in weight gain between the two groups.
Observational design: advantages: less costly and more feasible than an
experimental study since it would not involve assigning subjects to eat a particular
type of food. Experimental study: advantages: more control over what and how
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much participants are eating; disadvantages problems with compliance, more labor
intensive and costly for researcher.
B. LEVELS OF PREVENTION
1.
Prostate specific antigen (PSA) test for a 65-year-old man during his annual
physical exam:
Secondary prevention since the activity is aimed at early detection of
prostate cancer in a man with no apparent symptoms of this disease.
2.
Use of earplugs by employees in an automobile plant with loud, assembly line
equipment:
Primary prevention since the use of earplugs is aimed at reducing the risk
factors for hearing damage.
3.
Ordering a fecal occult blood test in an asymptomatic 55-year old man:
Secondary prevention because it is intended to detect early stages of
colorectal cancer in a man with no apparent symptoms of colorectal
cancer.
4.
Prescribing hormone replacement therapy for a woman with a family history of
osteoporosis:
Primary prevention because the treatment is intended to reduce the risk
factors for osteoporosis.
5. Measles, mumps and rubella vaccination for a 12 month old child:
Primary prevention since the activity aims to remove susceptibility to
measles, mumps and rubella.
6. HIV test for a healthy, pregnant woman at one of her prenatal visits:
Secondary prevention since the activity is aimed at testing for the
presence of HIV antibodies in a woman with no apparent symptoms of the
infection.
7. Stress reduction class for office employees with high blood pressure:
Primary prevention since the aim is to prevent diseases for which high
blood pressure is a risk factor.
8.
Nutritional counseling for elementary school children:
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Primary prevention because the activity is aimed at reducing the risk
factors for diseases associated with poor nutrition.
9.
Allergy skin testing and desensitization shots for an asthmatic 10-year old:
Tertiary prevention since it is intended to prevent future severe attacks that
keep child from being fully active or cause death associated with asthma
(decrease severity and complications in a child with chronic disease).
10.
Speech and occupational therapy for a 60 year old following a stroke:
Tertiary prevention since the activity is aimed at preventing complications
associated with having a stroke.
11.
Prescribing a cholesterol-lowering drug to a man recovering from a heart attack:
Tertiary prevention since the activity is aimed at preventing future heart
attacks in a man who already has a chronic disease.
12.
Conducting an annual skin test for TB in a medical student:
Secondary prevention since the activity is aimed at detecting non-clinical
TB infection in a patient who has a high risk of exposure to TB, so it can be
treated before it becomes active TB.
13.
Conducting a skin test for TB in a nonsmoker presenting with a persistent
cough:
None since this activity is being used as part of the diagnostic work up for a
patient who has symptoms of TB.
14.
Recommending that a computer programmer with an injured back use
an ergonometrically adjustable chair and computer station:
Tertiary prevention since the activity is intended to prevent future back
problems in a person who already has an injury.
15.
Mammogram for 45-year-old woman with breast lump discovered during self
breast exam.
None since mammography is being used to assist with the diagnostic workup for a breast lump.
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Questions
There will be 25 questions:
Approximately 18 will be multiple choice
with 4-6 answers
The remainder will be short answer
Aiming to make it significantly shorter
than the Midterm.
Topics
.
The final covers the entire course:
Approx. 40% is material covered prior
to Midterm
Approx. 60% is material covered SINCE
Midterm
Review strategy
1. Review homework (without looking at
answers)
Write notes for note sheet as you go
2. Write out answers to questions, dont
just think them
.
Review strategies
3. Focus on concepts, not formulas
(e.g. PVP)
4. Focus on the issues were about to go
over (all from Objectives)
5. Re-read objectives to date to reassure
yourself you have everything to date
Exam strategies
If you cant do a question, leave it and
come back to it (brain might be
triggered by later questions)
Remember your old friend the 4-fold
table (use it to help you answer
questions)
Examining concepts
Focused review
Construct a matched 2×2 table from
matched data, and compute a
matched odds ratio.
Interpret p-values and confidence
intervals associated with estimates of
relative risk, and recognize statistical
significance from either.
Focused review
Define, recognize, compute and
interpret the following:
attributable risk (AR),
attributable risk percent (AR%),
population attributable risk (PAR), and
population attributable risk percent
(PAR%).
Focused review
Recognize how changes in the
prevalence of an exposure in a
population affects relative risk,
attributable risk percent, and population
attributable risk percent.
Focused review
Define, recognize, compute and
interpret the following:
preventable risk (PR),
preventable risk percent (PR%),
absolute risk reduction (ARR),
relative risk reduction (RRR), and
number needed to treat (NNT) to prevent
one case.
Focused review
When given data from an observational
study:
(a) construct an appropriate 2 x 2 table (if
needed),
(b) compute the best estimate of the relative
risk, and
(c) interpret the result.
Focused review
Know how causal association can be
established
identify the criteria (i.e. guidelines) that
support a causal inference
recognize examples of these criteria or
guidelines.
Focused review
Recognize
potentially confounding variables
potential sources of selection bias and
measurement bias
differential vs. non-differential
misclassification.
Focused review
Recognize characteristics of a disease
that would make it an appropriate
target disease for a screening program.
Focused review
List, explain, and recognize the
common sources of bias that often
occur in studies designed to evaluate
the efficacy of a screening program
lead time bias
length bias
self-selection bias
overdiagnosis
Focused review
Recognize situations in which it would be
advantageous to use a screening test
with:
a high sensitivity (and low specificity),
a high specificity (and low sensitivity).
II. Key Epidemiologic Concepts
C. Natural History of Disease
2. Levels of Prevention
Levels of
Prevention
Primary
Definition:
Reducing
Risk Factors
Aims:
To Prevent Disease To improve
onset
outcome with
early treatment
Prevention
Modes of
Health promotion;
Intervention: immunization
Secondary
Prevention
Screening for
Early Detection
Tertiary
Prevention
Reducing Chronic
Disease
To limit Disability
and Delay
Progression
Screening plus
Rehabilitation;Resto
early diagnosis & rative surgery
treatment
Table B-Computation of Age-Adjusted Rates
1960
1960
1990
1990
Standard
Population
(1970)
Death
Rate
per 1000
Expected #
of Cases
Death
Rate
per 1000
Expected # of
Cases
I
J=C
0-14
226,452
.717
162.37
.316
71.56
15-24
187,509
1.124
210.76
1.000
187.51
25-44
276,810
1.834
507.67
1.762
487.74
45-64
196,411
9.624
1890.26
8.252
1620.78
65-84
102,901
40.435
4160.80
38.671
3979.28
9,917
181.916
1804.06
171.797
1703.71
.
Age Group
85+
Total
1,000,000
Computation
of AA Rates
K=JxI/1000
8735.92
8735.92/1,000,000
=.0087692=8.74 per 1000
L=F
M=JxI/1000
8050.58
8050.58/1,000,000
=.008050=8.05 per 10
I. Using Epidemiology to
determine the cause of Disease
A. Two-Step Process
2.
Is the association causal?
a.
Rule out alternative explanations
b.
Evaluate criteria for establishing
causality
Temporal relationship
biologic plausability
Strength of association
alternative explanation
Dose-response relationship
consistency with other data
Replication of findings
Hierarchy of Designs
S TUDY DES IGNS
DESCRIPTIVE
EXPLANATORY
Exam ine s assoc ia tions.
Te sts hy pothe se s.
De sc ribe s only . Does
not exam ine assoc iations.
EXPERIMENTAL
OBSERVATIONAL
Inve stiga tor obse rves
without inte rvention.
Inve stiga tor a ssigns
e xposure s.
Randomized
Clinical Trial
COHORT
CASE-CONTROL
CROSS-SECTIONAL
ECOLOGICAL
Cohort Studies
Then follow to see if
Totals Incidence
rates of
event
First
SELECT
Disease
occurs
Disease
does not
occur
Exposed
A
B
A+B
A/ (A+B)
Not
Exposed
C
D
C+D
C/(C+D)
Experimental Studies
Totals Incidence
rates of
event
Then
follow
to see if
Event
occurs
Event does
not occur
Exposed
A
B
A+B
A/ (A+B)
Not
Exposed
C
D
C+D
C/(C+D)
First
Assign
Case-Control Study
First SELECT on disease status
Cases
Lecture 14
Then
Measure
Controls
Exposed
A
B
Exposure Not
History
Exposed
C
D
Cross Sectional Studies
FIRST SELECT SAMPLE, then
MEASURE BOTH E AND O ON EVERY SUBJECT
IN THE SAMPLE.
Have
Disease
Do not
have
Disease
Totals
Exposed
A
B
A+B
Not Exposed
C
D
C+D
A+C
B+D
N
Totals
Proportion
A/ (A+C)
Exposed
B/(B+D)
Prevalence
Rates of
Disease
A/(A+B)
C/(C+D)
I.
1.
Attributable Risk
Difference
RATE DIFFERENCE. Computed as the difference
between two similar event rates, one being the rate
among the exposed and the other the rate among
the unexposed.
AR
II. Population Attributable Risk %
(PAR)
2. PROPORTION. Computed as a percent of the rate in
the entire population.
Preventable Risk
A.
PREVENTABLE RISK (PR and PR%):
PR
PR%
Interpretation: The PR is the number of events per unit of
population in the non-exposed population that could have
been prevented if they had been exposed to the protective
factor;
The PR% is the proportion of events in the non-exposed
population that could have been prevented if they had been
exposed to the protective factor.
Absolute Risk Reduction and
Relative Risk Reduction
B. RISK REDUCTION (ARR and RRR):
ARR
RRR
Interpretation: The ARR is the number of events
per unit of population that were prevented by the
experimental treatment;
the RRR is the proportion of events (in the control
group) that could have been prevented had they
received the experimental treatment.
Number Needed to Treat
C.
NUMBER NEEDED TO TREAT (NNT): The number
of individuals that would need to be treated (or
exposed to a protective factor) to prevent a single
event
Estimated by taking the reciprocal of the
Preventable Risk (PR) or Absolute Risk Reduction
(ARR).
NNT
= 1 / PR
or
1 / ARR
Similarly, the reciprocal of an Attributable Risk (AR) gives the
number of individuals exposed to a risk factor that would need
to have been unexposed to prevent a single event.
NNT
= 1 / AR
II. Lifetable Method
Time Interval
# at risk
#dying
#censored
0-1 year
22
3
0
1-2 years
19
0
0
2-3 years
19
0
0
3-4 years
19
0
0
4-5 years
19
1
1
5-6 years
17
1
0
6-7 years
16
0
0
7-8 years
16
3
0
8-9 years
13
0
0
9-10 years
13
0
0
10-11 years
13
0
3
11-12 years
10
1
2
12-13 years
7
1
2
13-14 years
4
1
1
14-15 years
2
0
2
II.
. Screening
Result
Positive
+
Negative
–
True
Validity
Diagnosis
Diseased Not Diseased
+
–
A
True +
C
False A+C
B
False +
D
TrueB+D
A+B
C+D
II.
Screening
Result
Validity
True
Diagnosis
Diseased
Not Diseased
Positive
A
True +
B
False +
A+B
Negative
C
False –
D
True –
C+D
A+C
B+D
PREDICTIVE VALUE POSITIVE (PVP) = A / (A+B)
a.
Prevalence of undetected disease increases (by increasing cell A)
A / (A+B)
b.
Specificity of the screening test increases (by decreasing cell B)
A / (A+B)
I.
The need to evaluate
screening programs
c.
Detectable preclinical phase = period prior to onset of Sx
when detection would be possible by a screening test
d.
Lead time = interval by which the time of diagnosis is
advanced by screening
II. An effective screening program must meet the
following conditions:
1. Important cause of mortality and morbidity.
Lecture 30
2. More amenable to treatment if diagnosed in
the presymptomatic stage than if it were
diagnosed later after symptoms arise.
3. The population being screened must have a
relatively high prevalence of the target
disease.
II. Threats to Internal Validity
Confounding
Vs
Bias
Both create FALSE associations
(or mask true associations)
3rd factor
Systematic error
Potentially
controllable
Fatal flaw
STEPS IN THE INVESTIGATION OF AN OUTBREAK OF
ACUTE DISEASE
1. Confirm the existence of an epidemic by
demonstrating that the previously reported
cases represent single entity present in
unexpectedly high numbers.
2. Describe the condition by identifying the
manifestations of the cases, and tentatively
choosing a diagnosis or a common set of
manifestations. [Outbreaks of disease that fit no
clear diagnostic category, even in retrospect,
are not uncommon.]
ETC
.
Fall 2017: Final
December 6, 2017
Instructions:
1. Write your name in the blank.
2. Circle your instructors name.
3. There are 6 questions. All questions have several parts.
4. Spaces for answers are provided below the questions in this single-sided 19-page document. Please put
all answers on this exam sheet in the spaces provided. Answers recorded elsewhere will not be considered.
5. If you need more room for calculations than what is provided in the answer blanks, you may use the back
of the previous page; make a note in the answer blank if you do so.
6. For all tests, use a significance level of D = 0.05 unless otherwise indicated.
7. For the statistical test questions, you must plug in the appropriate numbers for the test statistic to get full
credit. For example, on the line that says Work do not simply write the formula for the test statistic
again; you must plug in the appropriate numbers into the correct places in the formula.
8. Try to answer all questions, as you will not be penalized for wrong answers.
9. Be aware that not all questions are of equal value, so time yourself accordingly.
GOOD LUCK!
Page 2/19
Fall 2017: Final
December 6, 2017
A. (4 points each; 20 points total). For the following scenarios, identify the best statistical procedure the
researchers should use and briefly explain your choice.
x Independent (2-sample) t-test (equal
variances)
x Wilcoxon signed rank test
x Pearsons ?2-test
x Goodness-of-fit ?2-test
x r × c contingency table ?2-test
x Fishers Exact Test
x ANOVA F-test for completely randomized
designs (one-way ANOVA)
x Kruskal-Wallis test
x Pearson correlation coefficient
x Simple linear regression analysis
x Independent (2-sample) t-test (Welch)
x Paired (matched) t-test
x Wilcoxon rank sum test
x McNemars ?2-test
x Mantel-Haenszel ?2-test
x ANOVA F-test for randomized blocks
design
x Friedman-Kendall-Smith test
x Spearman correlation coefficient
x Logistic regression
x Log-rank test
x Cox proportional hazards regression
A.1. A study was conducted to follow overweight males, aged 2040 years of age for a period of 18 months.
During the study, three different methods of weight loss were assessed: calorie restriction alone, exercise
alone, and calorie restriction plus exercise. Participants were grouped into blocks defined by age-group.
Each block experienced all three weight loss programs, assigned in a random order within the block.
Total change in body weight was measured for each subject after 6 months. The investigators want to
test for differences in total change in body weight across the three weight loss programs. Histograms
revealed departure from normality for total change in body weight.
The best statistical procedure is:
(2 points)
Briefly explain choice of test:
(2 points)
A.2. In Nepal, a study was conducted to investigate whether exposure to indoor air pollution due to cooking
stove was associated with increased risk of cataract among rural women. Both the exposure and outcome
was measured as dichotomous variables (yes/no). In total, 128 women were recruited into the study, 34
with cataracts and 94 without cataracts.
The best statistical procedure is:
(2 points)
Briefly explain choice of test:
(2 points)
Page 3/19
Fall 2017: Final
December 6, 2017
A.3. A study was conducted in Wales relating blood pressure and lead levels in the blood. It was reported
that 40 out of 455 men with low lead levels (? 11?g/100mL) had elevated systolic blood pressure (SBP
? 160 mmHg), while 160 out of 410 men with high blood-lead levels had elevated SBP. For women, 60
out of 663 women with low blood-lead levels had elevated SBP, while 10 out of 192 women with high
blood-lead levels had elevated SBP. The researchers want to test the hypothesis that there is an
association between blood pressure and blood lead, while controlling for gender.
The best statistical procedure is:
(2 points)
Briefly explain choice of test:
(2 points)
A.4. A group of 12 hemophiliacs, all 40 years of age or younger at HIV seroconversion, was followed from
the time of primary AIDS diagnosis (between 1984 and 1989) until either death or the end of the study
in 1991. Another group of 10 hemophiliacsall of whom were older than 40 years of age at the time of
seroconversionwas followed independently in the same way. For all subjects, transmission of HIV
had occurred through infected blood products. The researchers would like to determine if the subjects
who were younger at seroconversion tended to live longer than the subjects who were older than 40 at
seroconversion.
The best statistical procedure is:
(2 points)
Briefly explain choice of test:
(2 points)
Page 4/19
Fall 2017: Final
December 6, 2017
A.5. The following table contains results looking at difference in baseline characteristics between people with
or without age-related macular degeneration (AMD). Name the statistical procedure that was used to
get the p-values marked by A.5.1, A.5.2, A.5.3 and A.5.4. (Assume normality for both age and diastolic
blood pressure, equal variances for age, and unequal variances for diastolic blood pressure.)
Variables
Age (years)
No AMD
Incident AMD
(N = 3808)
(N = 100)
Socio-demographic Characteristics
p-value
54.3 (±10.1)
60.8 (±12.2)
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