Slide 1

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 themthat will test my hypothesis? If my causal hypothesis is correct, how would the exposure andoutcome be distributed in these groups?

Enduring Understanding: Causal hypotheses can be tested by conducting investigations of theexposures and outcomes of selected groups of people as they go about their lives. Informationfrom these observational studies can be used to determine if an exposure and an outcome areassociated. Because observational studies are complicated by factors not controlled by theobserver, when an association is found, other explanations in addition to causality also must beconsidered.

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

Lesson 2-2: Cross-Sectional Studies

Teacher Note: Enduring Epidemiological Understandings forthe Epidemiology and the Energy Balance Equation Curriculum

1.Health and disease are not distributed haphazardly in apopulation. There are patterns to their occurrence that can beidentified through surveillance. Analysis of the patterns of healthand disease distribution can provide clues for formulatinghypotheses about their possible causes.

2.Causal hypotheses can be tested by conducting investigationsof the exposures and outcomes of selected groups of people asthey go about their lives. Information from these observationalstudies can be used to determine if an exposure and anoutcome are associated. Because observational studies arecomplicated by factors not controlled by the observer, otherexplanations also must be considered.

Lesson 2-2: Cross-Sectional Studies

Teacher Note: Authentic Assessment for Module 2 of theEpidemiology and the Energy Balance Equation Curriculum

Students will conduct, analyze, and interpret observational, cross-sectional studies among students in their class and then amongstudents outside their class. Working in teams, students will havethe opportunity to demonstrate their abilities to select areasonable hypothesis of interest to them, design study questionsabout exposure and outcome, obtain informed consent, collectand manage data, calculate and compare prevalence rates, makeaccurate statements about whether their data support thathypothesis, and consider alternate explanations for what theyobserved. Reporting of results will be required, such as a writtenreport, an item for the school newspaper, or an oral presentationor poster for students, teachers, and/or parents. Specificperformance criteria will be used to help ensure that theexperiences allow a genuine, realistic, and fair assessment ofstudents’ comprehension of the Module 2 EnduringEpidemiological Understanding.

Lesson 2-2: Cross-Sectional Studies

Start of Lesson 2-2

(estimate 1 class period)

Lesson 2-2: Cross-Sectional Studies

Big Ideas in Lesson 2-1

•Two things are associated whenthey “turn up together”

•Epidemiologists study associations.

•Descriptive epidemiology generates hypotheses aboutassociations

•Analytical epidemiology tests hypotheses aboutassociations

•A common way to test hypotheses is with anobservational study of a natural experiment

•The 2x2 table is a useful tool for numerically expressingassociations between exposure and outcome in a groupof individuals

Review

Lesson 2-2: Cross-Sectional Studies

The Journey from Exposure to Outcome

Lesson 2-2: Cross-Sectional Studies

Main Types of Analytical Epidemiology Studies

EXPERIMENTALManipulates whois exposed andthe exposure dose

Cohort Study

Case-Control Study

Cross-Sectional Study

OBSERVATIONALStudiesnaturalexperiments

Controlled Trial

Quasi-Experiment

Lesson 2-2: Cross-Sectional Studies

Exposed

Not Exposed

Outcome

NoOutcome

Total

IMPORTANT NOTICE!

Remember that any ofthe study types canuse the idea of a 2x2table to explore apossible associationbetween and exposureand an outcome

Lesson 2-2: Cross-Sectional Studies

Cross-Sectional Study

An observational study

A snapshotof whatis going on

Sometimes called aprevalence study

One pointin time

Lesson 2-2: Cross-Sectional Studies

What Do You Think?

•Would the quickest and easiest study be experimental orobservational?

Observational

•What would be the least amount of data you could collectfrom each study participant [in order to study anassociation]?

Two questions – one about the exposure, and oneabout the outcome

Lesson 2-2: Cross-Sectional Studies

Hypothesis: Students Who Bring Their Lunch From HomeAre Likely To Eat More Fruits and Vegetables

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What is the exposurein this hypothesis?

What is the outcomein this hypothesis?

Lesson 2-2: Cross-Sectional Studies

Using Data From a Real Cross-Sectional Study

School Lunchbehavior

NumberofStudents

Lunch from home4-5 times/week

718

Lunch from home1-3 times/week

543

Never broughtlunch from home

1,479

Total # ofStudents inSurvey

2,740

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home4-5 daysper week

5 or moreservings offruits andvegetablesper day

Total

718

339

<5 servingsof fruitsandvegetablesper day

Hypothesis: Students Who Bring Their Lunch From HomeAre Likely To Eat More Fruits and Vegetables

379

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Lesson 2-2: Cross-Sectional Studies

Review as Needed - Concepts of Prevalence

The numerator is the number ofpeople in the population orsample who experienced theoutcome.

The denominator is the totalnumber of people in thepopulation or sample.

Population /Sample

Outcome

Lesson 2-2: Cross-Sectional Studies

CHIS study - Express It In Numbers

Students who bring theirlunch to school from home4-5 days per week ANDeat 5 or more servings offruits and vegetablesper day

All students who bringtheir lunch to school fromhome 4-5 days per week

Numerator

Denominator

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Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home4-5 daysper week

5 or moreservings offruits andvegetablesper day

Total

718

339

<5 servingsof fruitsandvegetablesper day

Hypothesis: Students Who Bring Their Lunch From HomeAre Likely To Eat More Fruits and Vegetables

718

47%

TestedPrevalence Rateof eating 5 ormore servingsper day

339

379

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Lesson 2-2: Cross-Sectional Studies

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

Inference

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home4-5 daysper week

5 or moreservings offruits andvegetablesper day

Total

718

339

<5 servingsof fruitsandvegetablesper day

Hypothesis: Students Who Bring Their Lunch From HomeAre Likely To Eat More Fruits and Vegetables

718

47%

Prevalence Rateof eating 5 ormore servingsper day

339

379

What does this tell you about the hypothesis?

NOTHING!

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home4-5 daysper week

5 or moreservings offruits andvegetablesper day

Total

718

339

<5 servingsof fruitsandvegetablesper day

Hypothesis: Students Who Bring Their Lunch From HomeAre Likely To Eat More Fruits and Vegetables

718

47%

TestedPrevalence Rateof eating 5 ormore servingsper day

339

379

To study an association between an exposure and anoutcome, we need to compare prevalence of the outcomeamong people with the exposure to prevalence of theoutcome among people without the exposure.

Lesson 2-2: Cross-Sectional Studies

People who are observed in a studybut do not have the exposure

People whose results are comparedto the group that has the exposure

Definition of a Comparison Group

Lesson 2-2: Cross-Sectional Studies

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Total

718

339

379

339

718

47%

PrevalencePrevalence ofeating 5 ormore servingsper day

? %

Including a Comparison Group

a b

c d

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Extend and label the table to include a comparison group.

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

339

718

47%

PrevalencePrevalence ofeating 5 ormore servingsper day

? %

Including a Comparison Group

a b

c d

Lesson 2-2: Cross-Sectional Studies

Who would you choose as the comparison group?

SchoolLunchbehavior

NumberofStudents

Lunch fromhome 4-5times/week

718

Lunch fromhome 1-3times/week

543

Never broughtlunch fromhome

1,479

Total # ofStudents inSurvey

2,740

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Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

339

718

47%

PrevalencePrevalence ofeating 5 ormore servingsper day

1,479

? %

Never bringlunch fromhome

Including a Comparison Group

a b

c d

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“Never bringing lunch from home “ can meaneating in the school cafeteria or away from school,or not eating lunch at all

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

339

718

47%

1,479

Never bringlunch fromhome

Results of Actual Study

953

526

a b

c d

Prevalence ofeating 5 ormore servingsper day

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

339

718

47%

1,479

526

1,479

36%

Never bringlunch fromhome

Results of Actual Study

953

526

a b

c d

Prevalence ofeating 5 ormore servingsper day

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

PrevalencePrevalence ofeating 5 ormore servingsper day

1,479

Never bringlunch fromhome

Actual Study ResultsPrevalence and Prevalence Ratio

953

526

a b

c d

339

718

47%

526

36%

1,479

PrevalenceRatio

1.31

Lesson 2-2: Cross-Sectional Studies

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

Prevalence ofeating 5 ormore servingsper day

1,479

Never bringlunch fromhome

Actual Study ResultsPrevalence and Prevalence Ratio

953

526

a b

c d

339

718

47%

526

36%

1,479

PrevalenceRatio

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 comparedto students who never bring their lunch from home.

1.3

Bring lunchfrom home 4-5days per week

Lesson 2-2: Cross-Sectional Studies

Results

Interpretation

Actual study

Prevalence ratio of1.3 (positiveassociation)

Prevalence ratio above 1.0 means that theprevalence rate among the exposed group isgreater than the prevalence rate among theunexposed group

Interpretation of Prevalence Ratios

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

PrevalencePrevalence ofeating 5 ormore servingsper day

1,479

Never bringlunch fromhome

What if comparison numbers had been different?Hypothetical Analysis # 1

784

695

a b

c d

339

718

47%

695

47%

1,479

PrevalenceRatio

1.00

Students who bring their lunch from home 4-5 days per week are 1.0 timesas likely to eat 5 or more servings of fruits and vegetables compared tostudents who never bring their lunch from home.

Lesson 2-2: Cross-Sectional Studies

Results

Interpretation

Actual study

Prevalence ratio of 1.3(positive association)

Prevalence ratio above 1.0 means thatthe prevalence rate among the exposedgroup is greater than the prevalence rateamong the unexposed group

Hypothetical Study #1Prevalence ratio of 1.0(no association)

Prevalence ratio of 1.0 means that theprevalence rate among the exposedgroup is the same or similar as amongthe unexposed group

Interpretation of Prevalence Ratios

Lesson 2-2: Cross-Sectional Studies

Bring lunchfrom home 4-5days per week

5 or moreservings offruits andvegetablesper day

Total

718

339

379

<5 servingsof fruitsandvegetablesper day

PrevalencePrevalence ofeating 5 ormore servingsper day

1,479

Never bringlunch fromhome

What if comparison numbers had been different?Hypothetical Analysis # 2

444

1,035

a b

c d

339

718

47%

1,010

67%

1,479

PrevalenceRatio

0.70

Students who bring their lunch from home 4-5 days per week are 0.70 timesas likely to eat 5 or more servings of fruits and vegetables compared tostudents who never bring their lunch from home.

Lesson 2-2: Cross-Sectional Studies

Results

Interpretation

Actual study

Prevalence ratio of 1.3(positive association)

Prevalence ratio above 1.0 means thatthe prevalence rate among the exposedgroup is greater than the prevalence rateamong the unexposed group

Hypothetical Study #1Prevalence ratio of 1.0(no association)

Prevalence ratio of 1.0 means that theprevalence rate among the exposedgroup is the same or similar as amongthe unexposed group

Hypothetical Study #2

Prevalence ratio of 0.70

(negative association).

Prevalence ratio below 1.0 means thatthe prevalence rate among the exposedgroup is lower than the prevalence ratioamong the unexposed group

These examples show how prevalence ratios are impacted by theprevalence 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

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Egg

Chicken

Another Issue in Interpretation: Which came first?

Lesson 2-2: Cross-Sectional Studies

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 nextquestion is “compared to what?”

•The “compared to what” consists of prevalence for the unexposedcomparison group

• Dividing one prevalence by the other produces the prevalence ratio; it tells usif/how the exposure and outcome are associated

• Because exposure and outcome are measured at the same point in time, it isusually not possible to determine the time order of the exposure and theoutcome (which came first) and as such, the association may not beinterpretable

Re-Cap of Big Ideas So Far . . .

Lesson 2-2: Cross-Sectional Studies

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

Lesson 2-2: Cross-Sectional Studies

How is a real study reported?

Theresa A. Hastert, Susan H. Babey. Schoollunch source and adolescent dietary behavior.Prevention of Chronic Diseases 2009; Vol 6 (4)

http://www.cdc.gov/pcd/issues/2009/oct/08_0182.htm

OPTIONAL – may be bestsuited for high school

Lesson 2-2: Cross-Sectional Studies

Methods

“We analyzed cross-sectional data for 2,774adolescents who responded to the 2005 CaliforniaHealth Interview Survey (CHIS) and reported dietarybehaviors for a weekday.”

•Identify key information about the study presentedin 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 Studies

Results

“In bivariate analyses, adolescents who typically brought theirlunch from home 5 days per week ate fast food on feweroccasions; consumed fewer servings of soda, fried potatoes,and high-sugar foods; and ate more fruit and vegetablescompared with adolescents who never brought their lunch toschool. In linear regressions controlling for demographics,body mass index, desire to change weight, parent education,and adult presence after school, students who typicallybrought their lunch to school 5 days per week ate fast food0.35 fewer times and consumed 0.35 fewer servings of soda,0.10 fewer servings of fried potatoes, 0.25 fewer servings ofhigh-sugar foods, and 0.95 more servings of fruit andvegetables per day compared with students who never broughttheir lunch to school.”

Lesson 2-2: Cross-Sectional Studies

Results

Lesson 2-2: Cross-Sectional Studies

Results

“In bivariate analyses, adolescents who typically brought theirlunch from home 5 days per week ate fast food on feweroccasions; consumed fewer servings of soda, friedpotatoes, and high-sugar foods; and ate more fruit andvegetables compared with adolescents who never broughttheir lunch to school. In linear regressions controlling fordemographics, body mass index, desire to change weight,parent education, and adult presence after school, studentswho typically brought their lunch to school 5 days per week atefast food 0.35 fewer times and consumed 0.35 fewerservings of soda, 0.10 fewer servings of fried potatoes,0.25 fewer servings of high-sugar foods, and 0.95 moreservings of fruit and vegetables per day compared withstudents who never brought their lunch to school.”

Lesson 2-2: Cross-Sectional Studies

Table 2. Dietary Intake as a Function of Days Bringing Lunch to School AmongCalifornia Adolescents Aged 12-17 Years, California Health Interview Survey, 2005

Lesson 2-2: Cross-Sectional Studies

Table 2. Dietary Intake as a Function of Days Bringing Lunch to School AmongCalifornia Adolescents Aged 12-17 Years, California Health Interview Survey, 2005

Lesson 2-2: Cross-Sectional Studies

Table 2. Dietary Intake as a Function of Days Bringing Lunch to School AmongCalifornia Adolescents Aged 12-17 Years, California Health Interview Survey, 2005