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Reversed Time Order Is the association causal?
RR = 7 In Investigation 3-5: Reversed Time Order, students will uncover another explanation for why an epidemiologist might find an association. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Where are we and where are we going?
1 How is this disease distributed? Hypotheses 2 Is there an association? 3 Is the association causal? Why did the exposure and the disease turn up together? What’s my hypothesis? Did the exposure and the disease turn up together? 4 What should be done to prevent the disease? Remind students again that in the Module 3 investigations, they are learning how to answer the third Essential Question: “Is the association causal?” Next Slide 5 Did the prevention strategy work? Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
1. Cause 2. Chance 3. Confounding 4. Reversed Time Order In Module 3, students are exploring five explanations for why two things turn up together. In Investigation 3-2: Cause, they examined the possibility that an exposure and an outcome turn up together because the exposure causes the outcome. In Investigation 3-3: Chance, they explored the possibility that an exposure and an outcome turn up together by chance. In Investigation 3-4: Confounding, they explored the possibility that an exposure and an outcome turn up together as a result of confounding. In Investigation 3-5: Reversed Time Order, they will look at another reason why an exposure and an outcome may turn up together: reversed time order. Next Slide 5. Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Scenario 1 Ask students to consider the following scenarios: Scenario 1: There is a new student in kindergarten, a girl named Shakira. She loves animals and has four pet dogs, three cats, and seven rabbits. She talks of nothing but animals. Her friends say that she must have decided to have many pets because she loves animals. But which came first? Maybe she learned to love animals because her family had so many pets. ☼ Teacher Alert: Scenarios 1, 2, and 3 introduce the idea of reversed time order on a personal level that students should be able to relate to. Scenarios 4, 5, and 6 reinforce the concept of reversed time order in light of the various epidemiologic study designs. Students will recognize how some study designs are better than others in providing information about the time order of two things. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Scenario 2 Scenario 2: Your brother Sam sits close to the TV when he watches it. He has poor eyesight. Your mother claims that sitting too close to the TV has hurt his eyes. Sam claims that he sits close to the TV because he cannot see well when he sits farther back. Who is right? In other words, which came first, Sam’s poor eyesight or sitting too close to the TV? Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Scenario 3 Scenario 3: Your next door neighbor has a drinking problem and she is very depressed. Your father says that the drinking has caused her depression and that if she would stop, she would feel better. But you wonder about reversed time order. Maybe your neighbor was depressed and this started her drinking. Which came first, the drinking or the depression? Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Epi Speak Epi Speak Reversed Time Order When the hypothesized time order of an exposure and an outcome is actually reversed and the outcome really comes before the exposure. Ask students to find “Reversed time order” in the Epi Speak list. Review its definition. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Which came first? Exposure or Outcome?
Time Random Assignment - E O Study Design: Trial Healthy People Time - E O Study Design: Cohort Study Healthy People Time - O E Study Design: Case-Control Study Time - O E Study Design: Cross-Sectional Study Students should now consider how certain they are about the time order of the hypothesized exposure and outcome with each of these study designs. Ask students: ■ In which of these four study designs are you most certain that the hypothesized exposure came before the outcome? (Trial) ■ In which of these four study designs are you least certain that the hypothesized exposure came before the outcome? (Cross-sectional study) Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
1 Epi Log Worksheet Give each student an Investigation 3-5: Epi Log Worksheet. Students should complete Part 1 on their Investigation 3-5: Epi Log Worksheets by ranking the four study designs according to the certainty with which they know that the hypothesized exposure came before the outcome. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Which came first? Exposure or Outcome?
1 Which came first? Exposure or Outcome? 1st 2nd Time Random Assignment - E O Study Design: Trial Healthy People Time - E O Study Design: Cohort Study Healthy People 3rd 4th Time - O E Study Design: Case-Control Study Time - O E Study Design: Cross-Sectional Study (Trial, cohort study, case-control study, and cross-sectional study) ☼ Teacher Alert: In a prevention trial, you begin with people who are free of the disease (outcome) and then expose them to something that you hypothesize will keep them from getting the outcome in the future. If the outcome occurs, you can be certain that the exposure came before the outcome. (Remember, in a prevention trial, we do not want the outcome to occur.) In a cohort study, you begin with people who are free of the disease (outcome) and then observe whether or not they come in contact with the exposure that you hypothesize will cause the outcome in the future. If the outcome occurs, you can be certain that the exposure came before the outcome. In a case-control study, you begin with people who have the outcome (cases) and those who do not (controls) and ask about previous contact with the exposure that you hypothesize caused the outcome. If you are uncertain when the outcome occurred, as is the case with cancer, it is possible the time order is reversed and the “outcome” actually came before the exposure. In a cross-sectional study, you collect data about the outcome and the exposure (that you hypothesize caused the outcome) at the same time. Because of this, a cross-sectional study tells you nothing about the time order of the exposure and the outcome; it is possible the time order is reversed and the “outcome” actually came before the exposure. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Scenario 4 Watching TV No Watching TV Overweight No Overweight Scenario 4: This scenario is a hypothetical cross-sectional study testing the hypothesis “watching TV causes people to be overweight.” To test this hypothesis, students went to a high school and asked all 54 students in a freshman English class whether or not they watched TV. At the same time, they measured each student’s height and weight. Then, on the basis of sex, height, and weight, each student was defined as being overweight or not being overweight. ☼ Teacher Alert: Reinforce that the hypothesized exposure and outcome are being measured at the same point in time. This is the hallmark of a cross-sectional study. Perhaps overweight people watched more TV because they did not feel like getting up and doing something more active. Next Slide Watching TV Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Count, Divide, and Compare (CDC)
Is there an association? Count Over-weight No Over-weight Total Risks Relative Risk = 23 TV No TV 23 9 32 or 71.8% 32 b d a c 2.3 7 7 15 22 or 31.8% 22 Compare Divide Continuing with the hypothetical study, students counted, divided, and compared (CDC). They drew and labeled a 2 x 2 table. They counted the number of students who did and did not watch TV and, depending on whether or not they were overweight, placed them in one of the four cells of the 2 x 2 table. Next they divided to determine the risk of being overweight among students who watched TV and among those who did not. They then compared the risks of being overweight in the two groups by dividing the risk among those who watched TV (71.8%) by that among those who did not (31.8%) and calculating a relative risk (2.3). Ask students: ■ Did watching TV and being overweight turn up together? (Yes) ■ Did you find an association? (Yes) Next Slide Did watching TV and being overweight turn up together? Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Time Order Watching TV causes people to be overweight. Watching TV
No Watching TV Watching TV causes people to be overweight. Overweight No Overweight Remind students of the hypothesis: “watching TV causes people to be overweight.” Ask students: ■ Did the results of this cross-sectional study tell you anything about the time order of TV watching and being overweight? (No) ☼ Teacher Alert: Students may not understand this immediately. Help them realize that when we observe two things at the same point in time, we cannot be sure which came first. Next Slide Watching TV Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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X Time Order Watching TV causes people to be overweight. Watching TV
No Watching TV Watching TV causes people to be overweight. Overweight No Overweight Ask students: ■ If TV watching and being overweight occurred in the time order depicted in this slide, that is, watching TV came before being overweight, what might you suggest to get people to lose weight? (Tell them to stop watching TV.) Next Slide X Watching TV Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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X Time Order Watching TV causes people to be overweight. Watching TV
No Watching TV Watching TV causes people to be overweight. Overweight No Overweight Ask students: ■ What would happen if you convinced overweight people to stop watching TV but it was not TV watching that led to being overweight? Rather, being overweight led to watching TV. Then what would happen to the weight of the people who stopped watching TV? (Nothing, because watching TV was not causing people to be overweight; being overweight was causing people to watch TV.) Next Slide X Watching TV Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Watching TV causes weight gain. Scenario 5
No Watching TV Cigarette Smoking No Cigarette Smoking Overweight No Overweight Depression No Depression Scenario 5: This scenario is a cross-sectional study testing another hypothesis: “cigarette smoking causes depression.” To test this hypothesis, students went to a high school and asked all 376 students in several large senior history classes whether or not they smoked cigarettes and, at the same time, whether or not they were depressed. Next Slide Watching TV Cigarette Smoking Depression Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Count, Divide, and Compare (CDC)
Is there an association? Count Depress-ion No Depress-ion Cigarette Smoking No Cigarette Smoking Total b d a c 121 Risks or 31.4% 38 23.1% 255 59 Relative Risk = 1.4 38 83 59 196 121 255 Compare Divide Continuing with the hypothetical study, students counted, divided, and compared (CDC). They drew and labeled a 2 x 2 table. They counted the number of students who did and did not smoke and, depending on whether or not they were depressed, placed them in one of the four cells of the 2 x 2 table. They then divided to determine the risk of being depressed among students who smoked and among students who did not smoke. They then compared the risks of being depressed in the two groups by dividing the risk among those who smoked cigarettes (31.4%) by that among those who did not (23.1%). This is a calculation of relative risk, 1.4 in this example. It means that in this sample, depression is 1.4 times as likely among those who smoke cigarettes as among those who do not. Ask students: ■ Did cigarette smoking and depression turn up together? (Yes) ■ Did you find an association? (Yes) Next Slide Did cigarette smoking and depression turn up together? Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Time Order Watching TV causes weight gain.
No Watching TV Cigarette Smoking No Cigarette Smoking Cigarette smoking causes depression. Overweight No Overweight Depression No Depression Ask students: ■ In spite of the fact that you hypothesized that cigarette smoking caused depression, did the results of this cross-sectional study tell you anything about the time order of cigarette smoking and depression? (No) Next Slide Watching TV Cigarette Smoking Depression Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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X Time Order Cigarette smoking causes depression. Watching TV
No Watching TV Cigarette Smoking No Cigarette Smoking Cigarette smoking causes depression. Overweight No Overweight Depression No Depression Ask students: ■ If cigarette smoking and being depressed occurred in the time order depicted in this slide, what might you suggest to stop people from being depressed? (Tell them to stop smoking.) Next Slide X Watching TV Cigarette Smoking Depression Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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X Time Order Cigarette smoking causes depression. Watching TV
No Watching TV Cigarette Smoking No Cigarette Smoking Cigarette smoking causes depression. Overweight No Overweight Depression No Depression Ask students: ■ What would happen if you convinced depressed people to stop smoking but the time order of cigarette smoking and being depressed was actually reversed? (Nothing, because smoking was not causing people to be depressed; being depressed was causing people to smoke.) Next Slide X Watching TV Cigarette Smoking Depression Overweight Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Scenario 6 Scenario 6: Emily’s friend Jamal was in a fight on the school playground at lunch time. All afternoon, “Who won the fight?” was the talk of the school. But Emily was more interested in why her friend had been in the fight to begin with. She knew that Jamal’s family had recently bought a computer and that Jamal liked to play video games, especially violent ones. She wondered if these factors might have been responsible for Jamal’s fight. So, after school Emily found a computer in the school library, went to a search engine, typed in “Video Games and Violence,” hit “Enter,” and started surfing. One of the articles that caught her eye was a press release titled “Violent Video Games Can Increase Aggression.” Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Violent Video Games Emily printed the press release and started to investigate further. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Violent Video Games Emily clicked on the link to the journal article and began to read. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Violent Video Games Emily read the following: “On April 20, 1999, Eric Harris and Dylan Klebold launched an assault on Columbine High School in Littleton, Colorado, murdering 13 and wounding 23 before turning the guns on themselves. Although it is impossible to know exactly what caused these teens to attack their own classmates and teachers, a number of factors probably were involved. One possible contributing factor is violent video games. Harris and Klebold enjoyed playing the bloody, shoot-'em-up video game Doom, a game licensed by the U.S. military to train soldiers to effectively kill. The Simon Wiesenthal Center, which tracks Internet hate groups, found in its archives a copy of Harris' web site with a version of Doom that he had customized. In his version, there are two shooters, each with extra weapons and unlimited ammunition, and the other people in the game can't fight back. For a class project, Harris and Klebold made a videotape that was similar to their customized version of Doom. In the video, Harris and Klebold dress in trench coats, carry guns, and kill school athletes. They acted out their videotaped performance in real life less than a year later.” From Craig A. Anderson and Karen E Dill, “Video Games and Aggressive Thoughts, Feelings, and Behavior in the Laboratory and in Life,” Journal of Personality and Social Psychology, 78(April 2000): 772–790. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Violent Video Games Emily read on: “Entertainment media affects our lives. What behaviors children and adults consider appropriate comes, in part, from the lessons we learn from television and the movies. There are good theoretical reasons to expect that violent video games will have similar, and possibly larger, effects on aggression. The empirical literature on the effects of exposure to video game violence is sparse, however, in part because of its relatively recent emergence in modern U.S. society. About 25 years ago, when video games first appeared, popular games were simple and apparently harmless. In the 1970s, Atari introduced a game called Pong that was a simple video version of the game ping pong. In the 1980s, arcade games like Pac-Man became dominant. In Pac-Man, a yellow orb with a mouth raced around the screen chomping up ghosts and goblins. At this point, some eyebrows were raised questioning whether young people should play such 'violent’ games.” From Craig A. Anderson and Karen E Dill, “Video Games and Aggressive Thoughts, Feelings, and Behavior in the Laboratory and in Life,” Journal of Personality and Social Psychology, 78(April 2000): 772–790. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Violent Video Games Emily read further: “In the 1990s the face of video games changed dramatically. The most popular video game of 1993 was Mortal Kombat. This game features realistically rendered humanoid characters engaging in battle. As the name of the game implies, the goal of the player in Mortal Kombat is to kill any opponent he faces. Unfortunately, such violent games now dominate the market. One researcher sampled 33 popular Sega and Nintendo games and found that nearly 80% of the games were violent in nature. Interestingly, she also found that 21% of these games portrayed violence towards women.” From Craig A. Anderson and Karen E Dill, “Video Games and Aggressive Thoughts, Feelings, and Behavior in the Laboratory and in Life,” Journal of Personality and Social Psychology, 78(April 2000): 772–790. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Violent Video Games Can Increase Aggression
No Violent Video Games Aggression No Aggression As Emily kept reading, she learned that the study generating the press release was a cross-sectional study. She read that 227 volunteer college students completed questionnaires about both their exposure to violent video games and their aggressive behavior at the same point in time. Next Slide Violent Video Games Aggression Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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x Violent Video Games Can Increase Aggression
“Playing violent video games often may well cause increases in aggressive behavior.” No Violent Video Games Aggression No Aggression x The investigators concluded that “Playing violent video games often may well cause increases in aggressive behavior.” That is, the investigators assumed that the playing of violent video games came first and the aggressive behavior followed. Ask students: ■ If this were the case, how would you prevent aggressive behavior? (Take away violent video games) Next Slide Violent Video Games Aggression Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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x Violent Video Games Can Increase Aggression
No Violent Video Games “It could be that … highly aggressive individuals are especially attracted to violent video games.” Aggression No Aggression x The investigators warned, “It could be that … highly aggressive individuals are especially attracted to violent video games.” In other words, the hypothesized time order may be reversed and the association found because aggressive people like to play violent video games. That is, the aggressive behavior came first and playing violent video games started after. Ask students: ■ What would happen if you took away violent video games in an effort to reduce aggressive behavior, but the actual explanation for why the association was found was because aggressive children are more likely to play violent video games? What would happen to aggressive behavior? (Nothing) ■ Do you think it is important to consider the possibility of reversed time order when you find an association? Probe. Next Slide Violent Video Games Aggression Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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In the News Municipalities Can Limit Access to Violent Video Games
4 / 29 / 2002 A federal judge ruled that local governments should have the authority to limit children's access to violent video games, the Associated Press reported April 24. The ruling was made in a case brought by a video-game industry group against a St. Louis (Mo.) County ordinance. The ordinance, passed in 2000, regulates access to arcade and home video games. It requires children under 17 to have parental consent before they can purchase or play violent or sexually explicit games. Implementation of the ordinance was suspended until July 1 after the Interactive Digital Software Association sued the county. In making his ruling, Senior U.S. District Judge Stephen Limbaugh determined that the video games are not constitutionally protected forms of speech. Now have students to read this column from Join Together Online ( Ask students: ■ What do the people who proposed these rules think about the time order of playing violent video games and aggressive behavior? (Playing violent video games comes before aggressive behavior; playing violent video games causes aggressive behavior.) ■ Do the results of cross-sectional studies tell you about the time order of the exposure and the outcome? (No) ■ What would happen if these rules actually were effective in preventing children from playing violent video games, but the actual explanation for why the association was found was because aggressive children are more likely to play violent video games? What would happen to aggressive behavior? (Nothing) Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Which came first? Exposure No Exposure Outcome No Outcome Tell students to think about an association between an exposure and an outcome for which the time order might be reversed. Next Slide ? Exposure Outcome Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
2 Epi Log Worksheet Students should complete Part 2 on their Investigation 3-5: Epi Log Worksheets by identifying an association between an exposure and an outcome for which the time order might be reversed. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
2 Reversed Time Order It is possible that people who __________ are more likely to ____________. But it is also possible that people who are __________ are more likely to __________. watch TV be overweight overweight watch TV Ask students: ■ Can you please judge the quality of your work by trying to complete the following sentence? It is possible that people who __________ (watch TV) are more likely to __________ (be overweight), but it is also possible that people who are __________ (overweight) are more likely to __________ (watch TV). Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Epi Teams Divide the class into Epi Teams of four or five students per team. Students should now share and compare their answers to Part 2 with the other members of their Epi Team and edit their answers as needed. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Ask each Epi Team to select one association and write it on the board. Now ask a member of each team to explain this example of an association that may be due to reversed time order. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
3-6 Epi Log Worksheet Students should complete Parts 3 to 6 on their Investigation 3-5: Epi Log Worksheets by identifying other examples of associations between exposures and outcomes for which the time order might be reversed. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Intro to Epidemiology – Investigation 3-5: Reversed Time Order
Ask each Epi Team to select one association and write it on the board. Now ask a member of each team to explain this example of an association that may be due to reversed time order. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Explanations for Finding an Association
Could the association have been found because of reversed time order? RR = 7 Tell students that when they find, read, or hear about an association, they should think about it carefully. A good detective will not jump to the conclusion that the association is causal and will consider other explanations, including the possibility that the association was found as a result of reversed time order. Ask students: ■ What would you do to determine if an association was due to reversed time order? (Test the hypothesis using a cohort study or trial) Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Which came first? Exposure or Outcome?
Time Random Assignment - E O Study Design: Trial Healthy People Time - E O Study Design: Cohort Study Healthy People Time - O E Study Design: Case-Control Study Time - O E Study Design: Cross-Sectional Study ☼ Teacher Alert: In a prevention trial, you begin with people who are free of the disease (outcome) and then expose them to something that you hypothesize will keep them from getting the outcome in the future. If the outcome occurs, you can be certain that the exposure came before the outcome. (Remember, in a prevention trial, we do not want the outcome to occur.) In a cohort study, you begin with people who are free of the disease (outcome) and then observe whether or not they come in contact with the exposure that you hypothesize will cause the outcome in the future. If the outcome occurs, you can be certain that the exposure came before the outcome. In a case-control study, you begin with people who have the outcome (cases) and those who do not (controls) and ask about previous contact with the exposure that you hypothesize caused the outcome. If you are uncertain when the outcome occurred, as is the case with cancer, it is possible the time order is reversed and the “outcome” actually came before the exposure. In a cross-sectional study, you collect data about the outcome and the exposure (that you hypothesize caused the outcome) at the same time. Because of this, a cross-sectional study tells you nothing about the time order of the exposure and the outcome, and it is possible the time order is reversed and the “outcome” actually came before the exposure. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Explanations for Finding an Association
Study Links Eating M&M’s To Decreased Risk of Flu 1. 2. 3. 4. 5. Cause Chance Confounding Reversed Time Order Why would an exposure and an outcome turn up together? Students know from Investigation 3-2 that one of the possible explanations for finding an association between an exposure and an outcome―that is, they turn up together―is that the exposure causes the outcome. The exposure actually produces the effect of the outcome. Point out that when an association is found, the causal explanation is often the first one we consider. But students have also learned to consider the possibility that an association is found because of chance (Investigation 3-3) or confounding (Investigation 3-4). In Investigation 3-5, students have learned about a fourth possible explanation for why an epidemiologist might find a tie, link, relationship, or an association between an exposure and an outcome: reversed time order. Next Slide Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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Investigation 3-5 has ended.
Reversed Time Order Investigation has ended. CDC This concludes Investigation 3-5: Reversed Time Order and students can now put away their Epi Logs. Intro to Epidemiology – Investigation 3-5: Reversed Time Order Investigation 3-5
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