Date post: | 03-Jan-2016 |
Category: |
Documents |
Upload: | athena-snyder |
View: | 18 times |
Download: | 0 times |
Lesson 2-2: Cross-Sectional Studies
Teacher Note: Module 2 Overview
Content Area: Hypothesis-Testing: Cross-Sectional Study
Essential Questions: How can I select groups of people and collect data/evidence from them that will test my hypothesis? If my causal hypothesis is correct, how would the exposure and outcome be distributed in these groups? Enduring Understanding: Causal hypotheses can be tested by conducting investigations of the exposures and outcomes of selected groups of people as they go about their lives. Information from these observational studies can be used to determine if an exposure and an outcome are associated. Because observational studies are complicated by factors not controlled by the observer, when an association is found, other explanations in addition to causality also must be considered.
Core Concepts:• Testing hypotheses• Association• Exposure/outcome• Comparison group• 2x2 table• Observational studies• Study design/study plan• Study samples• Prevalence rate• Prevalence ratio• Statement of effect
Lessons:
2-1 Looking for Associations
2-2 Cross-Sectional Studies
2-3 Developing Hypothesis and Study Questions
2-4 Respect – Part II
2-5 Planning Study Conduct
2-6 Cross-Sectional Study - In Class
2-7 Cross-Sectional Study – In School
Revised Sept 14, 2011 1
Lesson 2-2: Cross-Sectional Studies
Teacher Note: Enduring Epidemiological Understandings for the Epidemiology and the Energy Balance Equation Curriculum
1. Health and disease are not distributed haphazardly in a
population. There are patterns to their occurrence that can be identified through surveillance. Analysis of the patterns of health and disease distribution can provide clues for formulating hypotheses about their possible causes.
2. Causal hypotheses can be tested by conducting investigations of the exposures and outcomes of selected groups of people as they go about their lives. Information from these observational studies can be used to determine if an exposure and an outcome are associated. Because observational studies are complicated by factors not controlled by the observer, other explanations also must be considered.
2
Lesson 2-2: Cross-Sectional Studies
Teacher Note: Authentic Assessment for Module 2 of the Epidemiology and the Energy Balance Equation Curriculum
3
Students will conduct, analyze, and interpret observational, cross-sectional studies among students in their class and then among students outside their class. Working in teams, students will have the opportunity to demonstrate their abilities to select a reasonable hypothesis of interest to them, design study questions about exposure and outcome, obtain informed consent, collect and manage data, calculate and compare prevalence rates, make accurate statements about whether their data support that hypothesis, and consider alternate explanations for what they observed. Reporting of results will be required, such as a written report, an item for the school newspaper, or an oral presentation or poster for students, teachers, and/or parents. Specific performance criteria will be used to help ensure that the experiences allow a genuine, realistic, and fair assessment of students’ comprehension of the Module 2 Enduring Epidemiological Understanding.
Lesson 2-2: Cross-Sectional Studies
Start of Lesson 2-2
(estimate 1 class period)
4
Lesson 2-2: Cross-Sectional Studies
Big Ideas in Lesson 2-1
• Two things are associated when they “turn up together”
• Epidemiologists study associations.
• Descriptive epidemiology generates hypotheses about associations
• Analytical epidemiology tests hypotheses about associations
• A common way to test hypotheses is with an observational study of a natural experiment
• The 2x2 table is a useful tool for numerically expressing associations between exposure and outcome in a group of individuals
Review
5
Lesson 2-2: Cross-Sectional Studies
The Journey from Exposure to Outcome
6
Lesson 2-2: Cross-Sectional Studies 7
Main Types of Analytical Epidemiology Studies
EXPERIMENTAL Manipulates who is exposed and the exposure dose
Cohort Study
Case-Control Study
Cross-Sectional Study
OBSERVATIONAL Studies
natural experiments
Controlled Trial
Quasi-Experiment
Lesson 2-2: Cross-Sectional Studies8
Exposed
Not Exposed
OutcomeNo
Outcome Total
IMPORTANT NOTICE!
Remember that any of
the study types can
use the idea of a 2x2
table to explore a
possible association
between and exposure
and an outcome
Lesson 2-2: Cross-Sectional Studies 9
Cross-Sectional Study
An observational study
A snapshot of what is going on
Sometimes called a
prevalence study
One point in time
Lesson 2-2: Cross-Sectional Studies10
What Do You Think?
• Would the quickest and easiest study be experimental or observational?
Observational
• What would be the least amount of data you could collect from each study participant [in order to study an association]?
Two questions – one about the exposure, and one about the outcome
Lesson 2-2: Cross-Sectional Studies
Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables
11
What is the exposure in this hypothesis?
What is the outcome in this hypothesis?
Lesson 2-2: Cross-Sectional Studies 12
Using Data From a Real Cross-Sectional Study
School Lunch behavior
Number of
Students
Lunch from home 4-5 times/week 718
Lunch from home 1-3 times/week 543
Never brought lunch from home 1,479
Total # of Students in Survey
2,740
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339
<5 servings of fruits
and vegetables
per day
Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables
13
379
Lesson 2-2: Cross-Sectional Studies
Review as Needed - Concepts of Prevalence
14
The numerator is the number of people in the population or sample who experienced the outcome.
The denominator is the total number of people in the population or sample.
Population / Sample
Outcome
Lesson 2-2: Cross-Sectional Studies
CHIS study - Express It In Numbers
Students who bring their lunch to school from home 4-5 days per week AND eat 5 or more servings of fruits and vegetables per day
All students who bring their lunch to school from home 4-5 days per week
Numerator
Denominator
15
+
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339
<5 servings of fruits
and vegetables
per day
Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables
16
718or 47%
Tested Prevalence Rate
of eating 5 or more servings
per day
339379
Lesson 2-2: Cross-Sectional Studies
Process of predicting from what is observed in a sample, to what is true for the entire population.
Inference
17
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339
<5 servings of fruits
and vegetables
per day
Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables
18
718or 47%
Prevalence Rate of eating 5 or
more servings per day
339379
What does this tell you about the hypothesis?
NOTHING!
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339
<5 servings of fruits
and vegetables
per day
Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables
19
718or 47%
Tested Prevalence Rate
of eating 5 or more servings
per day
339379
To study an association between an exposure and an outcome, we need to compare prevalence of the outcome
among people with the exposure to prevalence of the outcome among people without the exposure.
Lesson 2-2: Cross-Sectional Studies
People who are observed in a study but do not have the exposure
People whose results are compared to the group that has the
exposure
Definition of a Comparison Group
20
Lesson 2-2: Cross-Sectional Studies
Total
718339 379339
718or 47%
Prevalence Prevalence of
eating 5 or more servings
per day
?
?or ? %
Including a Comparison Group
21
a b
c d
Extend and label the table to include a comparison group.
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day Total
718339 379
<5 servings of fruits
and vegetables
per day
339
718or 47%
Prevalence Prevalence of
eating 5 or more servings
per day
?
?or ? %
Including a Comparison Group
22
a b
c d
?
Lesson 2-2: Cross-Sectional Studies 23
Who would you choose as the comparison group?
School Lunch
behavior
Number of Students
Lunch from home 4-5 times/week
718
Lunch from home 1-3 times/week
543
Never brought lunch from home
1,479
Total # of Students in Survey
2,740
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day Total
718339 379
<5 servings of fruits
and vegetables
per day
339
718or 47%
Prevalence Prevalence of
eating 5 or more servings
per day
1,479
?
1,479or ? %Never bring
lunch from home
Including a Comparison Group
24
a b
c d
“Never bringing lunch from home “ can mean eating in the school cafeteria or away from school,
or not eating lunch at all
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day Total
718339 379
<5 servings of fruits
and vegetables
per day
339
718or 47%
1,479Never bring lunch from
home
Results of Actual Study
25
953526
a b
c d
Prevalence of eating 5 or
more servings per day
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day Total
718339 379
<5 servings of fruits
and vegetables
per day
339
718or 47%
1,479
526
1,479or 36%Never bring
lunch from home
Results of Actual Study
26
953526
a b
c d
Prevalence of eating 5 or
more servings per day
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339 379
<5 servings of fruits
and vegetables
per day
Prevalence Prevalence of
eating 5 or more servings
per day
1,479Never bring lunch from
home
Actual Study Results Prevalence and Prevalence Ratio
27
953526
a b
c d
339718
or 47%
526 or 36%1,479
Prevalence Ratio
1.31
Lesson 2-2: Cross-Sectional Studies
5 or more servings of fruits and
vegetables per day
Total
718339 379
<5 servings of fruits
and vegetables
per day
Prevalence of eating 5 or
more servings per day
1,479Never bring lunch from
home
Actual Study Results Prevalence and Prevalence Ratio
28
953526
a b
c d
339718
or 47%
526 or 36%1,479
Prevalence Ratio
1.31
Students who bring their lunch from home 4-5 days per week are _____ times as likely to eat 5 or more servings of fruits and vegetables compared to students who never bring their lunch from home.
1.3
Bring lunch from home 4-5 days per week
Lesson 2-2: Cross-Sectional Studies29
Results Interpretation
Actual study Prevalence ratio of
1.3 (positive association)
Prevalence ratio above 1.0 means that the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group
Interpretation of Prevalence Ratios
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339 379
<5 servings of fruits
and vegetables
per day
Prevalence Prevalence of
eating 5 or more servings
per day
1,479Never bring lunch from
home
What if comparison numbers had been different? Hypothetical Analysis # 1
30
784695
a b
c d
339718
or 47%
695 or 47%1,479
Prevalence Ratio
1.00
Students who bring their lunch from home 4-5 days per week are 1.0 times as likely to eat 5 or more servings of fruits and vegetables compared to students who never bring their lunch from home.
Lesson 2-2: Cross-Sectional Studies31
Results Interpretation
Actual study Prevalence ratio of 1.3
(positive association)
Prevalence ratio above 1.0 means that the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group
Hypothetical Study #1 Prevalence ratio of 1.0 (no association)
Prevalence ratio of 1.0 means that the prevalence rate among the exposed group is the same or similar as among the unexposed group
Interpretation of Prevalence Ratios
Lesson 2-2: Cross-Sectional Studies
Bring lunch from home 4-5 days per week
5 or more servings of fruits and
vegetables per day
Total
718339 379
<5 servings of fruits
and vegetables
per day
Prevalence Prevalence of
eating 5 or more servings
per day
1,479Never bring lunch from
home
What if comparison numbers had been different? Hypothetical Analysis # 2
32
4441,035
a b
c d
339718
or 47%
1,010 or 67%1,479
Prevalence Ratio
0.70
Students who bring their lunch from home 4-5 days per week are 0.70 times as likely to eat 5 or more servings of fruits and vegetables compared to students who never bring their lunch from home.
Lesson 2-2: Cross-Sectional Studies33
Results Interpretation
Actual study Prevalence ratio of 1.3
(positive association)
Prevalence ratio above 1.0 means that the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group
Hypothetical Study #1 Prevalence ratio of 1.0 (no association)
Prevalence ratio of 1.0 means that the prevalence rate among the exposed group is the same or similar as among the unexposed group
Hypothetical Study #2 Prevalence ratio of 0.70 (negative association).
Prevalence ratio below 1.0 means that the prevalence rate among the exposed group is lower than the prevalence ratio among the unexposed group
These examples show how prevalence ratios are impacted by the prevalence rate in the comparison group. Without this information, the prevalence ratio of the exposed group is not interpretable.
Interpretation of Prevalence Ratios
Lesson 2-2: Cross-Sectional Studies
Egg Chicken
Another Issue in Interpretation: Which came first?
34
Lesson 2-2: Cross-Sectional Studies35
Big Ideas in Lesson 2-2
• The cross-sectional design is an observational study of a natural experiment
• This design is relatively quick and simple, asking individuals about exposure and outcome at one point in time
• When prevalence is calculated for the group with the exposure, the next question is “compared to what?”
• The “compared to what” consists of prevalence for the unexposed comparison group
• Dividing one prevalence by the other produces the prevalence ratio; it tells us if/how the exposure and outcome are associated
• Because exposure and outcome are measured at the same point in time, it is usually not possible to determine the time order of the exposure and the outcome (which came first) and as such, the association may not be interpretable
Re-Cap of Big Ideas So Far . . .
Lesson 2-2: Cross-Sectional Studies36
Optional session to explore how a real study is reported (the CHIS example)
Lesson 2-2: Cross-Sectional Studies
How is a real study reported?
Theresa A. Hastert, Susan H. Babey. School lunch source and adolescent dietary behavior. Prevention of Chronic Diseases 2009; Vol 6 (4)
http://www.cdc.gov/pcd/issues/2009/oct/08_0182.htm
37
OPTIONAL – may be best
suited for high school
Lesson 2-2: Cross-Sectional Studies38
Methods“We analyzed cross-sectional data for 2,774 adolescents who responded to the 2005 California Health Interview Survey (CHIS) and reported dietary behaviors for a weekday.”
•Identify key information about the study presented in this single-sentence description of the methods.
When did the survey take place?
What was the sample size?What study design was used?
Who were the study subjects?
What was the name of the survey?What topic was being studied?
Lesson 2-2: Cross-Sectional Studies39
Results“In bivariate analyses, adolescents who typically brought their lunch from home 5 days per week ate fast food on fewer occasions; consumed fewer servings of soda, fried potatoes, and high-sugar foods; and ate more fruit and vegetables compared with adolescents who never brought their lunch to school. In linear regressions controlling for demographics, body mass index, desire to change weight, parent education, and adult presence after school, students who typically brought their lunch to school 5 days per week ate fast food 0.35 fewer times and consumed 0.35 fewer servings of soda, 0.10 fewer servings of fried potatoes, 0.25 fewer servings of high-sugar foods, and 0.95 more servings of fruit and vegetables per day compared with students who never brought their lunch to school.”
Lesson 2-2: Cross-Sectional Studies40
Results“In bivariate analyses, adolescents who typically brought their lunch from home 5 days per week ate fast food on fewer occasions; consumed fewer servings of soda, fried potatoes, and high-sugar foods; and ate more fruit and vegetables compared with adolescents who never brought their lunch to school. In linear regressions controlling for demographics, body mass index, desire to change weight, parent education, and adult presence after school, students who typically brought their lunch to school 5 days per week ate fast food 0.35 fewer times and consumed 0.35 fewer servings of soda, 0.10 fewer servings of fried potatoes, 0.25 fewer servings of high-sugar foods, and 0.95 more servings of fruit and vegetables per day compared with students who never brought their lunch to school.”
Lesson 2-2: Cross-Sectional Studies41
Results“In bivariate analyses, adolescents who typically brought their lunch from home 5 days per week ate fast food on fewer occasions; consumed fewer servings of soda, fried potatoes, and high-sugar foods; and ate more fruit and vegetables compared with adolescents who never brought their lunch to school. In linear regressions controlling for demographics, body mass index, desire to change weight, parent education, and adult presence after school, students who typically brought their lunch to school 5 days per week ate fast food 0.35 fewer times and consumed 0.35 fewer servings of soda, 0.10 fewer servings of fried potatoes, 0.25 fewer servings of high-sugar foods, and 0.95 more servings of fruit and vegetables per day compared with students who never brought their lunch to school.”
Lesson 2-2: Cross-Sectional Studies42
Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged 12-17 Years, California Health Interview Survey, 2005
Lesson 2-2: Cross-Sectional Studies43
Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged 12-17 Years, California Health Interview Survey, 2005
Lesson 2-2: Cross-Sectional Studies44
Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged 12-17 Years, California Health Interview Survey, 2005