PowerPoint Presentation

DAY 4Considerations for Interpretation

Confounding1

Introduction

Understanding what the data can tell us and cannot tell us

23Approach to understanding the data If we see an association, what other possible explanations can there be besides causation?

- Confounding

ConfoundTo confuse4Websters Online Dictionary www.webster-dictionary.org

To mistake for another5

Guilt by associationAnother example of why two things might turn up together is guilt by association or what epidemiologists call confounding. In this cartoon, one of the boys is responsible for breaking the window , but he is running away (see arrow) and the boy standing below the window is being blamed. Just like in this cartoon, an exposure might be blamed for an outcome, when in fact a different exposure might be the real culprit. An example related to the energy balance equation might be blaming television or video game playing for childhood obesity when the real culprit is the reduced level of physical activity associated with such sedentary behaviors, or the increased consumption of junk foods while watching television or playing games. 5Not ObservedJamesCauseXObserved

Broken WindowGeorge

Guilt by association6 Who was blamed for drinking the beer? (Lennie) Why was Lennie blamed for drinking the beer? (When Coach Cully came upon the scene, he observed Lennie near the empty beer bottles) Why didnt Coach Cully think that Hugo drank the beer? (He did not observe Hugo) Who actually drank the beer? (Hugo) Why was Lennie, not Hugo, blamed for drinking the beer? (When Lennie and Hugo were briefly together behind the shed, they were associated with each other. Lennie was guilty by association.)

Scenario 1 Individual level

HugoLennie

7Scenario 1 involves Lennie and Hugo, two high school students on the baseball team. Late one spring afternoon, Lennie comes across Hugo behind a sports equipment shed at the back of the high school athletic field. He is surprised to see his friend sitting against the back of the shed, surrounded by several empty beer bottles and drinking the last remaining bottle. Hugo! exclaims Lennie. What in the world - you know you cant do this!!At the same moment, the boys hear approaching footsteps. Hugo jumps up and vaults over the fence and runs down the street. Lennie, too surprised to move, stands transfixed as Coach Cully rounds the corner of the shed. The rest is history - Lennie is put on probation and is kicked off the baseball team. Note: Scenario 1 introduces the idea of confounding on a personal level that most people can understand in terms of their own lives. Later, Scenario 2 reinforces the concept of confounding using a population example a hypothetical epidemiology study of the relationship between watching R-rated movies and teenage smoking and drinking.

HugoNot Observed Empty Beer Bottles

LennieCauseAssociationCauseXObserved

Scenario 1 Individual level

8Ask students: Who was blamed for drinking the beer? (Lennie) Why was Lennie blamed for drinking the beer? (When Coach Cully came upon the scene, he observed Lennie near the empty beer bottles) Why didnt Coach Cully think that Hugo drank the beer? (He did not observe Hugo) Who actually drank the beer? (Hugo) Why was Lennie, not Hugo, blamed for drinking the beer? (When Lennie and Hugo were briefly together behind the shed, they were associated with each other. Lennie was guilty by association.)

Scenario 2 Population level 9Scenario 2: Students are listening to the news, and they hear the anchorwoman say that the observational study showed that teenagers who watch R-rated movies were more likely to try alcohol than teenagers who do not watch R-rated movies. Although this Scenario is hypothetical, there have been studies of this association. For example, a study published in 2002 (citation below) examined relationships between parental restrictions on viewing R-rated movies and the likelihood of teenagers trying alcohol and cigarette smoking. Among 4,544 students in grades 5 through 8 enrolled in fifteen schools in Vermont and New Hampshire, the following results were reported: The prevalence of having tried smoking was 35% for those with no restrictions on viewing R-rated movies, 12% for those with partial restrictions, and 2% for those with complete restrictions. The prevalence of having tried alcohol as 46% for those with no restrictions on viewing R-rated movies, 16% for those with partial restrictions, and 4% for those with complete restrictions. Even after controlling for other factors, children who had no restrictions from viewing R-rated movies were significantly more likely to try smoking and alcohol compared with those who had complete restrictions. But, controlling for other factors (confounders) did reduce the magnitude of the associations between restrictions from reviewing R-rated movies and trying alcohol and smoking. Related actual study: MA Dalton, et. al., Relationship between Parental Restrictions on Movies and Adolescent Use of Tobacco and Alcohol, Effective Clinical Practice, January / February 2002, Volume 5, Number 1, Pages 1-10.

Try AlcoholNot Try Alcohol6Yes R-Rated MoviesNo R-Rated Movies90bdacPrevalencePrevalence Ratio

Scenario 2 Population level People who _____________ are ______times as likely

to __________ as people who ____________.

10 How would this evidence be presented in a 2 x 2 table? What is the exposure? (R-rated movies) What is the outcome? (Teenagers trying alcohol) What is the prevalence ratio? (6 - teenagers who watch R-rated movies are 6 times more likely to try alcohol than those who do not watch R-rated movies.

Trying Alcohol Watching R-Rated MoviesThe study establishes a clear progression that begins with watching R-rated movies and leads to teenagers trying alcohol.If we can keep our teenagers from watching R-rated movies, then we can go a long way towards preventing teenagers from trying alcohol.Cause

Scenario 2 Population level 11The anchorwoman then quoted the investigators: The study establishes a clear progression that begins with watching R-rated movies and leads to trying alcohol. The anchorwoman said that the investigators concluded, If we can keep our teenagers from watching R-rated movies, then we can go a long way towards preventing teenagers from trying alcohol. On the basis of the above quotes, why does the investigator think the association between watching R-rated movies and trying alcohol was found? (Cause)

? Trying Alcohol Watching R-Rated MoviesCauseAssociationCauseX

Scenario 2 Population level 12Review Confounding is possible when an unobserved exposure that is both associated with the observed exposure and is an actual cause of the outcome is called a confounder.

13

A situation in which an association between an exposure and an outcome is observed as a result of the influence of a third, unobserved, factor, called a confounder.

The true effect of an exposure on an outcome is distorted because it is mixed up with another factor that can influence the outcome.

The confounder is both associated with the observed exposure and is a possible cause of the outcome.Confounding13Moyses Szklo and F. Javier Nieto, Epidemiology: Beyond the Basics, Aspen Publishers, 2000.Unobserved ExposureTry alcoholWatch R-rated moviesLimited Adult SupervisionWatch out for lurking variables!

14If an association was really found due to confounding, and you avoided or eliminated the hypothesized cause, what would happen to the outcome. (If the association was found due to confounding, you would not prevent the outcome from occurring by avoiding or eliminating the hypothesized cause.)

Unobserved ExposureTry alcoholWatch R-rated moviesWillingness to Break the Rules15Unobserved ExposureTry alcoholWatch R-rated moviesPoor Self-Control16Unobserved ExposureTry alcoholWatch R-rated movies Parental attitudes17Unobserved ExposureTry alcoholWatch R-rated movies Risk taker18Lack of Supervision or Poor Self- Control or Risk taking or . . . Try alcoholWatch R-rated moviesX and you avoided or eliminated the hypothesized cause, what would happen to the outcome??If the association was due to confounding 19Ask students, if an association was really found due to confounding, and you avoided or eliminated the hypothesized cause, what would happen to the outcome. (If the association was found due to confounding, you would not prevent the outcome from occurring by avoiding or eliminating the hypothesized cause.)

CauseAssociationCauseXConfounderPlaying Volleyball Skin Cancer

20Go through each of the next nine slides by showing the observed exposure and the outcome, and then identify one or more possible confounders and complete the confounding diagram. (Because volleyball is often played on the beach, possible confounders include sun and suntan lotion. However, your response should not be limited to these suggestions. Keep in mind the definition of a confounder: an exposure that both is associated with the exposure being observed and is an actual cause of the outcome.)

CauseAssociationCauseXConfounderDrinking Alcohol Lung Cancer

21Smoking is often associated with other behaviors, and as such, is a potential confounder because it is a possible cause of many adverse outcomes.

CauseAssociationCauseXConfounderEating PretzelsAuto Accidents

22Because pretzels and other salty foods are often consumed when drinking alcohol, a possible confounder is drinking alcohol.

CauseAssociationCauseXConfounderWatching TVAcne

23Because watching TV is often associated with eating junk food, junk food is a possible confounder

CauseAssociationCauseXConfounderDrinking CoffeeCancer

24Because coffee drinking is often accompanied by other exposures, possible confounders include a sweetener, newspapers, donuts, and Styrofoam cups. CauseAssociationCauseXConfounderDriving MotorcyclesHepatitis C

25Because driving motorcycles is associated with having tattoos and tattoos can be drawn with dirty needles, a possible confounder is dirty needles.

CauseAssociationCauseXConfounderPlaying Baseball Oral Cancer

26Because baseball players often chew tobacco or eat sunflower seeds, these exposures are possible confounders

CauseAssociationCauseXConfounderSand Skin Cancer

27Because people who go to the beach are not only being exposed to sand but also to the sun, the sun is a possible confounder.

Association is not necessarily causation.

Association is not necessarily causation.

Association is not necessarily causation.

Association is not necessarily causation.

Association is not necessarily causation.

Association is not necessarily causation.

Association is not necessarily causation. Association is not necessarily causation.Always Remember28Are all associations causal? (No)Are all causal relationships associations? (Yes)

Supplementary Lesson about ConfoundingConfounding: Working the Numbers29The next few slides provide a numerical explanation for confounding by showing in a simple way how a confounder needs to be associated with the exposure of interest and with the outcome of interest. The confounder is associated with the observed exposure (but not affected by the exposure).

The confounder is a possible cause of the outcome.

Two criteria for ConfoundingPossible ConfounderOutcomeExposure30The anchorwoman then quoted the investigators: The study establishes a clear progression that begins with watching R-rated movies and leads to trying alcohol. The anchorwoman said that the investigators concluded, If we can keep our teenagers from watching R-rated movies, then we can go a long way towards preventing teenagers from trying alcohol. On the basis of the above quotes, why does the investigator think the association between watching R-rated movies and trying alcohol was found? (Cause)

31We suspect a factor to be a potential confounder based on intuition and evidence that it is associated with the exposure under study and a cause of the outcome under study

But is this true in the data at hand?

We can assess this by calculating the strength of association between: 1) exposure and the suspected confounder; and 2) between suspected confounder and outcome. Possible ConfounderOutcomeExposurePR = ?PR = ?PR = ?324001000 40 %4006001000 a b c dExercise lessExercise moreTotalIncreased BPNo increased BPPrevalence2001000 20 %2008001000 Prevalence Ratio2.0 a a+b c c + d People who exercise less are more likely to have high blood pressure. People who exercise less are 2.0 times as likely to have increased blood pressure as those who exercise more. 200032CauseAssociationCauseXConfounder?Exercise less Increased blood pressure33Smoking Increased blood pressureExercise LessPR = ?PR = ?PR = 2.0Assessment of smoking as a potential confounder

See if it meets two criteria for confounding1. Associated with the exposure?2. A possible cause of the outcome?354501000 45.0 %4505501000 a b c dLess exerciseMore exerciseTotalSmokersNon-smokersPrevalence1751000 17.5 %1758251000 Prevalence Ratio2.6People who exercise less are 2.6 times as likely to be smokers as those who exercise more. 20001. Is the potential confounder associated with the exposure?(Is smoking associated with exercise?)35SmokingIncreased Blood PressureExercise LessPR = 2.6PR = ?PR = 2.037475625 76.0 %475150625 a b c dSmokersNon-smokersTotalIncreased BPNo increased BPPrevalence1251250 10.0 %12512501375 Prevalence Ratio7.6Smokers are 7.6 times as likely to have increased blood pressure compared to non-smokers. 20002. Is the potential confounder a possible cause of the outcome?(Is smoking a possible cause of increased blood pressure?)37SmokingIncreased Blood PressureExercise LessPR = 2.6PR = 7.6PR = 2.0Is there Possible Confounding?39Marijuana Use and Getting TattoosPeople who ____________________________________________ are ______ times as likely to _______________________________compared to people who __________________________________4050200 25 %50150200 a b c dMarijuanaUserNot a marijuana UserTotalTattoos No TattoosPrevalence30200 15 %30170200 Prevalence Ratio1.7 a a+b c c + d People who smoke marijuana are more likely to get tattoos than people who do not smoke marijuana. 40People who use marijuana are 1.7 times as likely to get tattooed compared to people who do not use marijuana.4150200 25 %50150200 a b c dMarijuanaUserNot a marijuana UserTotalTattoos No Tattoos Prevalence30200 15 %30170200 Prevalence Ratio1.7People who smoke marijuana are more likely to get tattoos than people who do not smoke marijuana. 41CauseAssociationCauseXConfounder?Use Marijuana Get Tattoos42Access to MoneyGet TattoosUse MarijuanaPR = ?PR = ?PR = 1.7Assessment of access to money as a potential confounder

See if it meets two criteria for confounding1. Associated with the exposure?2. A possible cause of the outcome?44150200 75.0 %15050200 a b c dTotalPrevalence50200 25.0 %50150200 Prevalence Ratio3.0People who use marijuana are 3.0 times as likely to have access to money than non-users. 400MarijuanaUserNot a marijuana UserAccess to Money No Access to Money 1. Is the potential confounder associated with the exposure?(Is access to money associated with marijuana use?)44Access to MoneyGet TattoosUse MarijuanaPR = 3.0PR = ?PR = 1.74660140 42.8%60140200 a b c dTotalPrevalence20180 11.1 %20180200 Prevalence Ratio3.9People with access to money are 3.9 times as likely to get tattoos than people without access to money. 400Access to MoneyNo Access to MoneyTattoos No Tattoos2. Is the potential confounder a possible cause of the outcome?(Is access to money a possible cause of getting tattoos?46Access to MoneyGet TattoosUse MarijuanaPR = 3.0PR = 3.9PR = 1.7Is there Possible Confounding?Possible ConfounderOutcomeExposurePR = 0.97 PR = 1.10 PR = 1.8 Associated with the exposure? A possible cause of the outcome?Another Example - Is there Possible Confounding?Day 4 First Morning Breakout

Assessing Possibility of Confounding4950Use the 54 cards in the deck to explore the potential for confounding in this data setDeck Worksheet pages 3-5

Less Frequent BreakfastHigh Physical ActivityHigh SleepPR = ? PR = ? PR = ?Associated with the exposure? A possible cause of the outcome?Assess Possibility of Confounding by Frequency of Breakfast

Assessing confounding in your study Assembly to brainstorm ideas . . . 52Day 4 Second Morning Breakout

Possible confounders you should examine?53Possible Confounder(s)?Your OutcomeYour ExposureDiscuss possible confounding in your study

Label your possible confounding triangle on large post-it Breakout Deliverable for Assembly/Gallery walk

55Large post-it with your teams confounding triangle (During gallery walk after the break, use small post-its to provide constructive comments and suggestions on other teams triangles).56Study Proposal: Section 7a

7. Considerations for Interpretation

7a. Confounding

Describe how a confounding variable could influence your hypothesized association

Develop one or two additional study questions to help assess possible confounding

If you have time, start this . . .57Breakout Session Tips Create one or two questions for your survey that will assess possible confounding. Your question(s) must be school appropriateLimit your answer options to two answers, such as Yes/No, Male/Female, or other 2-choice answers. Anything else?

5758Reminder

It should be written so that its intended audience understands it.

It should mean the same thing to everyone who reads it.

The answer options must categorize and cover the entire range of possible behavior

The time period to consider when answering a particular question must be appropriate (long or short enough)Criteria for a good survey question58