Day 3 Prevalence 1

Continuing with Tools for Doing the Study On an average school day, how many hours do you watch TV?   A. I do not watch TV on an average school day  B. Less than 1 hour per day  C. 1 hour per day  D. 2 hours per day  E. 3 hours per day  F. 4 hours per day G. 5 or more hours per day ✔ From questions to answers ✔ From answers to counts From counts to prevalence From prevalence to statements

Continuing with tools for doing a study ✔What are you curious about? ✔From curiosity to a hypothesis ✔From a hypothesis to questions ✔From questions to answers ✔From answers to counts From counts to prevalence From prevalence to statements Interpretation – Conclusions - Communication

Three main tools

Review - Tool # 1 Cross-sectional study design: a relatively quick way to test a hypothesis Sometimes called a prevalence study A snapshot of what is going on This is the best design for a high school project, because it is feasible in a shorter time frame and can be conducted in classes. It might not have the most thorough information collected, but is an appropriate way to start exploring a hypothesis. Has limitations that will test students ability to think critically. One point in time An observational study

Review - Tool # 2 2. Contingency table: puts numbers in a table so we can get from answers to counts Shows exposure and outcome The simplest table is the 2x2 table Everyone is in the table somewhere Handy for calculations

Tool # 3 3. Prevalence – calculations to quantify outcomes in populations; prevalence ratios (comparisons) provide a measure of association between exposure and outcome Calculated as a fraction or percentage Everyone with the outcome – recent and long-term Especially used in cross-sectional studies

A cross-sectional study is sometimes called a prevalence study. From Epi Textbooks The main outcome measure obtained from a cross-sectional study is prevalence. A cross-sectional study is sometimes called a prevalence study.

Prevalence The number of people with a specified condition or event, among a specified population and at a specified time The proportion of a population found to have a condition (typically a disease such as diabetes or a health-related behavior such as smoking or seat-belt use)

Express it in numbers The Numerator is the number of people in the population or sample who experienced the outcome or effect, in this case, wearing blue. The Denominator is the total number of people in the population or sample, in this case, total number of students in the class.

Prevalence of wearing blue Numerator The number of students who are wearing blue Denominator All the students in the class

Prevalence of wearing blue # in class x 100 = % wearing blue = Prevalence

Prevalence of wearing glasses The number of students who are wearing glasses Numerator Denominator All the students in the class

Prevalence of wearing glasses # in class x 100 = % wearing glasses = Prevalence

Numerator Denominator The number of students who had cereal for breakfast Denominator All the students in the class

Numerator Denominator The number of students who walked to school All the students in the class

Numerator Denominator The number of students who . . . ? All the students in the class

Prevalence Ratio A comparison of two prevalences Calculated by dividing the prevalence of the outcome in the exposed by the prevalence of the outcome in the unexposed   a/(a+b) divided by c/(c+d).

High school students who send more text messages/day are more likely to binge drink compared to students who send fewer text messages/day. Binge drinker Not a binge drinker Prevalence Ratio Prevalence Total 30 Hyper-texter 30 70 100 30 % 100 a b c d 1.4 Not a hyper-texter 88 88 312 400 22 % 400 ÷ a a+b c c + d People who ____________________________________________ are ______ times as likely to _______________________________ compared to people who __________________________________

High school students who send more text messages/day are more likely to binge drink compared to students who send fewer text messages/day. 30 100 30 % 70 a b c d Hyper-texter Not a hyper-texter Total Binge drinker Not a binge drinker Prevalence 88 400 22 % 312 Prevalence Ratio 1.4 Hyper-texters are 1.4 times as likely to binge drink than those who are not hyper-texters.

Interpretation of Prevalence Ratios Results Interpretation Prevalence Ratio Above 1.0 POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

Teenagers who are not restricted from watching R-rated films are more likely to try smoking compared to teenagers who have restrictions on watching R-rated films. Tried smoking Did not try smoking Prevalence Ratio Prevalence Total 656 No restriction 656 771 1427 46 % 1427 a b c d 3.5 Partial or complete restriction 413 413 2704 3117 13% 3117 ÷ a a+b c c + d People who ____________________________________________ are ______ times as likely to _______________________________ compared to people who __________________________________

Teenagers who are not restricted from watching R-rated films are more likely to try smoking compared to teenagers who have restrictions on watching R-rated films. 656 1427 46 % 771 a b c d No restriction Partial or complete restriction Total Tried smoking Did not try smoking Prevalence 413 3117 13% 2704 Prevalence Ratio 3.5 Teenagers who have no restrictions on watching R-rated films are 3.5 times as likely to try smoking as those who have restrictions.

Interpretation of Prevalence Ratios Results Interpretation Prevalence Ratio Above 1.0 POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

Students living in urban areas engage in more experimenting with prescription drugs than students living in rural areas. Did Experiment Did not experiment Prevalence Ratio Prevalence Total 95 Urban 95 905 1000 9.5 % 1000 a b c d 0.73 Rural 130 130 870 1000 13.0 % 1000 ÷ a a+b c c + d People who ____________________________________________ are ______ times as likely to _______________________________ compared to people who __________________________________

Students living in urban areas engage in more experimenting with prescription drugs than students living in rural areas. 95 1000 9.5 % 905 a b c d Urban Rural Total Experiment Did not experiment Prevalence 130 13.0 % 870 Prevalence Ratio 0.73 Students in urban areas are 0.73 times as likely to experiment with prescription drugs than students in rural areas.

Interpretation of Prevalence Ratios Results Interpretation Prevalence Ratio Above 1.0 POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group

Interpretation of Prevalence Ratios Results Interpretation Prevalence Ratio Above 1.0 POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group Prevalence Ratio Below 1.0 NEGATIVE ASSOCIATION the prevalence rate among the exposed group is lower than the prevalence rate among the unexposed group

Interpretation of Prevalence Ratios Results Interpretation Prevalence Ratio Above 1.0 POSITIVE ASSOCIATION - the prevalence rate among the exposed group is greater than the prevalence rate among the unexposed group Prevalence Ratio Below 1.0 NEGATIVE ASSOCIATION the prevalence rate among the exposed group is lower than the prevalence rate among the unexposed group Prevalence Ratio At or Near 1.0 NO ASSOCIATION – the prevalence rate among the exposed group is similar or the same as the prevalence rate among the unexposed group The interpretation of a prevalence ratio is: A prevalence ratio of 1.0 means that the outcome of interest occurs with the same frequency in the exposed and unexposed groups A prevalence ratio above 1.0 means that the outcome of interest occurs more among the exposed group than among the unexposed group A prevalence ratio below 1.0 means that the outcome of interest occurs less in the exposed group than among the unexposed group Also try to understand the strength of a study result. In other words, the further a prevalence ratio is from 1.0 (higher or lower), the stronger the association. For example, 1.1 is a weak positive association and 3.1 is a strongly positive association. Similarly, 0.95 is a weak negative association, while 0.45 is a strongly negative association. When an association is strong, we have more confidence that it is real. A prevalence ratio of 1.1 is a weak positive association, while a prevalence ratio of 3.1 is a strong positive association A prevalence ratio of 0.95 is a weak negative association, while a ratio of 0.45 is a strong negative association

Results from some Epi Teams in Paterson NJ Epi Stars - Drinking at least 2 cans or a 20-ounce bottle of non-diet soda every day leads to a crash (feeling tired) - PR = 2.5 Pop Science – A healthy breakfast is associated with playing in an organized sport - PR = 0.96 Hypertensions – Receiving a daily, weekly, or monthly allowance is related to eating junk food/unhealthy food more than twice a day - PR = 1.6 Dr. Observation – Healthy eating (at least 2 servings of fruit and vegetables a day) results in better grades (“doing well in school”) - PR = 1.0

Quick Summary of Cross-Sectional Study Calculations Questions about exposure and outcome are answered simultaneously. Answers on exposure and outcome can be put into a 2x2 table. A “yes/no” answer will fit If using a multiple choice question, a predetermined “cut point” is needed to define a “higher/lower” range to fit into a 2x2 table. Counts in the 2x2 table allow calculations of prevalence Comparisons of prevalences (prevalence ratio) allows a statement about the association between exposure and outcome.

No difference between exposed and unexposed Measure of Risk The prevalence ratio (PR) is a measure of risk used in cross-sectional studies. It compares prevalence in the exposed to prevalence in the unexposed. A ratio of 1.0 denotes no difference between the two groups. Interpreting PR and Confidence Intervals Prevalence Ratio No difference between exposed and unexposed Examples This slide is about statistical significance, which is beyond the scope of the Epi Challenge but may be of interest to some students. The vertical red line indicates a PR of 1 (unity) = no difference in outcome among the exposed versus the unexposed. The dark horizontal lines show the calculated PR (dot) and the width of the 95% confidence interval (95% probability that this interval contains the true SMR) Any line that is completely to the left or to the right of the red bar is a statistically significant result at p<0.05

Breakout Assignment Prevalence

Perform a few practice calculations as needed

Deck Worksheet – page 2 Calculate the prevalence of the outcome – for the exposed group and for the unexposed group. Calculate the prevalence ratio. Populate the 2x2 table on page 1 with the above information. Make a statement that uses the prevalence ratio to describe size of the association.

Study Proposal: Section 6 Data Analysis Plan 6a. Contingency Table 6b. Prevalence among exposed 6c. Prevalence among unexposed 6d. Prevalence Ratio 6e. Statement of results 6f. How prevalence ratio will be used in your study