Discussion:

Find a scientific journal article on a health study that contains an example of a causal criteria. Using a screen shot or cut/paste, post the section that demonstrates the criteria. But don't give away the answer! Include a link to the full article.

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Summary Table of Causal Criteria

Causal Criteria	Definition	What Is the Key Word/Phrase to Identify This Criteria?
Strength of the association
Consistency with other investigations
Temporality
Specificity of the association
Biological gradient (Dose-Response relationship)
Experimental evidence = epidemiological study designs
Biological Plausibility
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Causal Criteria

For each of the following sets of information in Questions 1 and 2, name the one main causal criterion described by Sir Bradford Hill that is best supported by the data. There may be more than one causal criterion, so select the one you feel is best represented. Provide a short explanation (2 grammatically correct sentences) for why you selected that specific criterion. You will lose points for not having an explanation or for poor grammar/sentence structure/ etc.As epidemiologists, good communication is essential!

See Friis pp. 138-139 for descriptions of the criteria.

1. Maternal Smoking

The following information is based on studies that examined the relationship between maternal smoking and adverse health outcomes.

a. A study in Switzerland was completed for all women giving birth during a 12-month period. There was a total of 6,284 singleton births. Information was collected on smoking during pregnancy, as well as information on whether the infant was low birthweight, small for gestational age or premature. Odds ratios were calculated for each of the levels of smoking. In this study, the odds ratio measures the association between the exposure (smoking) and the three outcomes (low birthweight, small for gestational age, preterm birth).

An odds ratio over 1.0 indicates a positive association between the exposure and outcome; one less than 1.0 indicates a negative (protective) association. An odds ratio of 1.5 - 2.9 suggests a moderate association between the exposure and outcome, while an odds ratio greater than 3.0 suggests a strong association.

	Odds Ratio (95% Confidence Interval)
Low birthweight	Small for gestational age	Preterm birth
Non-smoker	1.0	1.0	1.0
1-9 cigarettes/day	1.7 (1.2-2.6)	1.9 (1.5-2.5)	1.0 (0.7-1.6)
10-19 cigarettes/day	3.5 (2.4-5.0)	2.2 (1.7-2.9)	1.5 (1.0-2.3)
20 cigarettes/day	4.0 (2.5-6.3)	2.4 (1.7-3.5)	2.4 (1.5-3.9)

b. The lower birth weight of babies born to mothers who smoke has been documented for almost 40 years. Nicotine affects the developing fetus by reducing blood flow through the placenta which retards growth of the fetus and contributes directly to low birth weight of the baby.

c. A review of six studies conducted before 2000 suggested that smoking 1-10 cigarettes per day during pregnancy was associated with increased risk of preterm birth. Another analysis of 20 prospective studies of maternal smoking and preterm delivery in different North American and European populations found odds ratios varying from 1.16 to 1.31. Finally, a third study examined 8 case-control studies from around the world.Odds ratios for these studies ranged from 1.0 - 4.2.

d. Demographic characteristics and data on exposure to tobacco smoke during pregnancy were obtained from 3,227 infants of at least 34 weeks gestation. Of the 3227 neonates studied, 30.9% were exposed to maternal smoking during pregnancy. A decrease in birthweight and an increase in the prevalence of growth restriction with the increasing number of cigarette consumption was observed. This effect was evident even in cases of a "minimal" consumption of 1-5 cigarettes per day.

2. Childhood Asthma

The following information is based on studies that examined the relationship between childhood body mass index and subsequent physician-diagnosed asthma. Asthma is characterized by airway inflammation, enhanced airway responsiveness to a variety of environmental stimuli, and reversible airway obstruction.

For each set of information, list one causal criterion that is supported by the data and provide a short explanation of why you selected this criterion. There may be more than one causal criterion that applies to each set of information so pick the one you feel is best supported by the data.

a. Obesity can reduce lung volume, affect blood volume to the lung airways, and even affect how well a patient responds to asthma medication.

b. A meta-analysis (a method of reviewing and summarizing multiple studies) of many prospective epidemiologic studies found that the risk of adult incident asthma increases with increasing BMI in both men and women. Other studies suggest that a similar trend might be observed in children.

c. Six prospective cohort studies examined the association between being overweight/obese and being diagnosed with asthma in children <18 years of age. In boys, the risk of being diagnosed with asthma was 1.01 to 2.0 times higher in overweight/obese boys compared to normal weight boys. In girls, the risk of being diagnosed with asthma was up to 2.3 times higher in overweight/obese girls compared to normal weight girls.

d. A study by Gilliland et al. in 2003 followed 3,792 asthma-free children ages 7 - 18 years old for four years. The children's weight and height were measured at enrollment and each year they were asked if they had been newly diagnosed with asthma by a physician.Obese children were 1.6 times more likely to be newly diagnosed with asthma.

e. A proposed hypothesis is that a high body weight, as a state of low-grade inflammation, exacerbates airway inflammation, which contributes to the development of asthma.A typical Western diet may act as an inflammatory stimulus. If inflammation is a mechanism linking high body weight and asthma, it is reasonable that diet plays some role. A typical Western eating patternhigh in energy-dense foods such as animal fats and processed sugars, and low in whole unprocessed plant foodsmay be obesogenic due to its energy-dense foods.

f. A comprehensive literature search yielded 434 studies that examined the relationship between childhood obesity and development of asthma. Forty-eight studies were used in a further meta-analysis and systematic review. The authors concluded that these studies provide evidence to support a weak yet significant association between high body weight in children and asthma. (Papoutsakis et al. 2013)