Lesson 2-2: Cross-Sectional Studies Teacher Note: Module 2 Overview Content Area:...

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


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


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.


Start of Lesson 2-2

(estimate 1 class period)

4


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


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


Cross-Sectional Study

An observational study

A snapshot of what is going on

Sometimes called a

prevalence study

One point in time


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


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?


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


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


13

379


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


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

+




vegetables per day

Total

718339


and vegetables

per day


16

718or 47%

Tested Prevalence Rate

of eating 5 or more servings

per day

339379


Process of predicting from what is observed in a sample, to what is true for the entire population.

Inference

17




vegetables per day

Total

718339


and vegetables

per day


18

718or 47%

Prevalence Rate of eating 5 or

more servings per day

339379

What does this tell you about the hypothesis?

NOTHING!




vegetables per day

Total

718339


and vegetables

per day


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.


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


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.




vegetables per day Total

718339 379


and vegetables

per day

339

718or 47%



per day

?

?or ? %


22

a b

c d

?


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





718339 379


and vegetables

per day

339

718or 47%



per day

1,479

?

1,479or ? %Never bring

lunch from home


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





718339 379


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






718339 379


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






vegetables per day

Total

718339 379


and vegetables

per day



per day


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



vegetables per day

Total

718339 379


and vegetables

per day




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



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




vegetables per day

Total

718339 379


and vegetables

per day



per day


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.



Actual study Prevalence ratio of 1.3

(positive association)


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





vegetables per day

Total

718339 379


and vegetables

per day



per day


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.



Actual study Prevalence ratio of 1.3

(positive association)


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.



Egg Chicken

Another Issue in Interpretation: Which came first?

34


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 . . .


Optional session to explore how a real study is reported (the CHIS example)


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


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?


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.”






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





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