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, 20111

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 Studies 8 Exposed Not Exposed Outcome No 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 Studies 10 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/week718 Lunch from home 1-3 times/week543 Never brought lunch from home1,479 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 <5 servings of fruits and vegetables per day Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables

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 <5 servings of fruits and vegetables per day Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables or 47% Tested Prevalence Rate of eating 5 or more servings per day

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 <5 servings of fruits and vegetables per day Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables or 47% Prevalence Rate of eating 5 or more servings per day 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 <5 servings of fruits and vegetables per day Hypothesis: Students Who Bring Their Lunch From Home Are Likely To Eat More Fruits and Vegetables or 47% Tested Prevalence Rate of eating 5 or more servings per day 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 or 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 <5 servings of fruits and vegetables per day or 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 <5 servings of fruits and vegetables per day or 47% Prevalence Prevalence of eating 5 or more servings per day 1,479 ? or ? % 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 <5 servings of fruits and vegetables per day or 47% 1,479 Never bring lunch from home Results of Actual Study 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 <5 servings of fruits and vegetables per day or 47% 1, ,479 or 36% Never bring lunch from home Results of Actual Study 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 <5 servings of fruits and vegetables per day Prevalence Prevalence of eating 5 or more servings per day 1,479 Never bring lunch from home Actual Study Results Prevalence and Prevalence Ratio a b c d 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 <5 servings of fruits and vegetables per day Prevalence of eating 5 or more servings per day 1,479 Never bring lunch from home Actual Study Results Prevalence and Prevalence Ratio a b c d 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 Studies 29 ResultsInterpretation 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 <5 servings of fruits and vegetables per day Prevalence Prevalence of eating 5 or more servings per day 1,479 Never bring lunch from home What if comparison numbers had been different? Hypothetical Analysis # a b c d 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 Studies 31 ResultsInterpretation 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 <5 servings of fruits and vegetables per day Prevalence Prevalence of eating 5 or more servings per day 1,479 Never bring lunch from home What if comparison numbers had been different? Hypothetical Analysis # ,035 a b c d 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 Studies 33 ResultsInterpretation 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 EggChicken Another Issue in Interpretation: Which came first? 34

Lesson 2-2: Cross-Sectional Studies 35 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 Studies 36 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) 37 OPTIONAL – may be best suited for high school

Lesson 2-2: Cross-Sectional Studies 38 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 Studies 39 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 Studies 40 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 Studies 41 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 Studies 42 Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged Years, California Health Interview Survey, 2005

Lesson 2-2: Cross-Sectional Studies 43 Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged Years, California Health Interview Survey, 2005

Lesson 2-2: Cross-Sectional Studies 44 Table 2. Dietary Intake as a Function of Days Bringing Lunch to School Among California Adolescents Aged Years, California Health Interview Survey, 2005