Epiville

Causal Inference

Data Analysis

Being in the Attorney General's Office surrounded by all of those legal books, led you to think about how Hill's causal "criteria" (Click here for a link to the original article or see Aschengrau & Seage, pp 392-399) can be used to assess evidence for causality in much the same way that a prosecutor attempts to establish guilt in a court of law. Basically, epidemiologists have looked to lists of 'causal criteria' as inductive ways of building an argument to support the notion that a given association is causal. Your job is to use Hill's criteria to give the Attorney General guidance about whether the Gidwani et al article shows that television viewing is a cause of early initiation of smoking.

5. The authors state in the results, "Controlling for baseline characteristics, youth who watched >5 hours of television per day were 5.99 times more likely to initiate smoking behaviors than those youth who watched 0-2 hours per day." What does this piece of information tell us?

  1. Watching >5 hours of television per day causes youths to initiate smoking.
  2. There is a statistical association between watching >5 hours of television per day and initiation of smoking after taking into account other baseline characteristics of youth who watched television that were measured in this study.
  3. If kids watch less TV, they will be less likely to initiate smoking.
Answer (a) — incorrect: They are simply stating the statistical association that they found in their data; this information does not tell us if this association is causal.
Answer (b) — correct: They are stating that there is an association between the exposure and outcome, but cannot determine on the basis of these data alone whether this association is causal. They also report that their finding is "adjusted for" baseline differences between children who watch television and those who do not. Adjustment is a way to control for possible differences in the two comparison groups which could be independently associated with differences in television viewing patterns (confounding).
Answer (c) — incorrect: They are stating that there is an association between the exposure and outcome, but cannot determine on the basis of these data alone whether this association is causal. They also report that their finding is "adjusted for" baseline differences between children who watch television and those who do not. Adjustment is a way to control for possible differences in the two comparison groups which could be independently associated with differences in television viewing patterns (confounding).

6. The following quotes provide evidence to support Hill's Criteria. Choose the criterion which correctly matches each quote.

1. “We examined the relationship between television viewing and initiation of smoking and found a strong dose-response relationship with increasing hours.”
2. “In this study, television viewing was measured 2 years before smoking initiation.”
3. “The association was substantial, with youth who watched >5 hours per day being 5.99 times as likely to initiate smoking than youth who watched 0 to 2 hours per day.”
4. “…Television provides adolescents with role models, including movie and television stars and athletes, who portray smoking as a personally and socially rewarding behavior.”
5. “The findings are consistent with social learning theory.”
6. “A similar association between television viewing and the onset of alcohol use has been reported…”

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7. Which of the criteria that you reviewed above is the one that is essential for TV watching to be a cause of smoking initiation?

  1. Biological plausibility
  2. Dose-response
  3. Temporality
Answer (a) — incorrect: It is possible for a factor to be causal even if we do not know the exact biological mechanism by which it operates.
Answer (b) — incorrect: It is possible that there is a threshold effect of TV viewing, so that only after a certain length of viewing it is harmful.
Answer (c) — correct: By the definition of cause, the cause must precede the effect. Temporality is the only 'required' criterion in the evaluation of causality.

Intellectually curious?

Learn more about the limitations of causal criteria.

Hill's 'criteria' are often used by epidemiologists in an attempt to rule out alternative explanations for an association (other than a causal explanation). Epidemiologists also look to other explanations, those not explicitly covered in Hill's guidelines, when assessing whether an association is plausibly causal. You will learn more about these "alternative explanations" in the lectures on bias and confounding. Being the vigorous young epidemiologist that you are, you know that you should consider potential alternative explanations for the association between television viewing and smoking initiation before giving your report to the Attorney General.

8. Which of the following alternative explanations could have possibly caused the authors to find that youth who watched >5 hours of television per day were 5.99 times more likely to initiate smoking behaviors, if watching TV did not truly cause smoking initiation?

  1. If a true cause of smoking initiation was poor parental monitoring, and youth who watch a lot of TV are less likely to be monitored by parents, then the association between TV watching and smoking initiation would really be due to their shared association with parental monitoring. This is an example of confounding.
  2. Youth who watch TV are more likely to under-report true smoking habits on a survey compared to youth who do not watch TV.
  3. Youth who watch a lot of TV are more likely to have participated in the follow-up survey since they have more free time.
Answer (a) — correct: If the true cause was being home alone after school, and youth who watch TV are more likely to be home alone after school, then the association between TV watching and smoking initiation would really be due to their shared association with being home alone after school. This is an example of confounding.
Answer (b) — incorrect: If that were the case, then it would appear that TV-watchers are LESS likely to initiate smoking.
Answer (c) — incorrect: For this type of phenomenon to explain the results, the youth who watched a lot of television AND started smoking early would have to be disproportionately followed-up, compared to children who watched a lot of television and did not initiate smoking early.