09-exercises.htm 6/30/2010
Notes:
- It is often difficult to
determine from brief descriptions whether a study is based on person-level
or aggregate-level data. To help distinguish this element, look for hints
such as whether regional comparisons were made (aggregate-level) or
whether individuals were contacted individually.
- It is often difficult to
determine from brief descriptions whether observational data are
longitudinal or cross-sectional. You may thus confuse cross-sectional
studies and case-control studies. To help distinguish these designs, look
for hints such as selection of study participants based on disease status
(case-control), random selection of participants from a population without
regard for exposure or disease status (cross-sectional), or follow-up of
individuals over time (cohort study).
- Non-randomized trials are
similar in design to cohort studies, and may indeed be classified as such
by some researchers (e.g., Maclure, 1991). The
difference is that protocol of the non-randomized trial allocates the
exposure to individuals. In cohort studies, the investigator does not
interfere with the exposure status of individuals.
9.1 - 9.9 See text.
Read the passages below and
identify the study design as either (a) a randomized trial, (b) cohort study,
(c) case-control study, or (d) cross-sectional or ecological study. Justify your response.
9.10 Calorie restriction during childhood and prostate cancer risk later
in life. A study investigated the incidence of prostate cancer in 58,279
men between the ages of 55 and 69, with subjects classified according to
whether they had experienced caloric restrictions during childhood. Rates of
prostate cancer were compared in those with evidence of caloric restriction and
those that were not exposed to such deprivation (Am J Epidemiol, 2001;154:530-537).
9.11 Tea consumption in relation to CHD. The relation of tea to
cholesterol, systolic blood pressure, and mortality from coronary heart disease
and all causes was studied in 9,856 men and 10,233 women without history of
cardiovascular disease or diabetes. All men and women 35-49 years of age from
the county of Oppland (Norway) were invited to
participate; the attendance rate was 90%. RESULTS.
Mean serum cholesterol decreased with increasing tea consumption, the linear
trend coefficient corresponded to a difference of 0.24 mmol/liter (9.3 mg/dl)
in men and 0.15 mmol/liter (5.8 mg/dl) in women between drinkers of less than
one cup and those of five or more cups/day, when other risk factors were taken
into account. Systolic blood pressure was inversely related to tea with a
difference between the same two tea groups of 2.1 mm in men and 3.5 mm in
women. Altogether 396 men and 237 women died from all causes, and of these 141
and 18, respectively, died from coronary heart disease during the 12-year
follow-up period. The mortality rate was higher (not statistically significant)
among persons drinking no tea or less than one cup compared with persons
drinking one or more cups/day. This applies to men and women and to coronary
heart disease and all-cause mortality. For men, the relative risk (one or more
versus less than one cup) for coronary death from Cox regression was 0.64 (95%
CI: 0.38, 1.07).(Stensvold et al., Prev Med,
1992,21:546-53.).
9.12 Effectiveness of antibiotics in preventing post-op infections. Surgical
patients were randomly assigned to one of three groups. Group 1 received
antibiotics 24 hours prior to surgery, group 2 received antibiotics 2 hours
prior to surgery, and group 3 received no antibiotics prior to surgery.
Post-operative infection rates were determined and compared.
9.13 Folic acid study 1. Researchers in Liverpool, England
published a landmark study in which 66% of the mothers who had given birth to
infants with spinal cord malformations were folic acid deficient. In contrast,
mothers who had normal babies were shown to be folic acid deficient 17% of the
time (Lancet 1965; i, 1254 - 1256).
9.14 Folic acid study 2. The British Research Council
completed a study in which some of the women in a group who had given birth to
babies with neural tube defects in the past and who were once again pregnant
were supplemented with folic acid during pregnancy. The investigators found
that supplementation reduced the risk of having second neural tube defect baby
by 70% (Lancet 1991;338:131-137; PubMed).
9.15 Optimists and pessimists. At the Mayo Clinic in Rochester, Minnesota,
researchers studied 939 patients who had been classified as either optimists or
pessimists. After thirty years, a greater percentage of optimists were alive
than pessimists (Mayo Clinic Proceedings 2000;75:140-143).
9.16 Community survey. A random sample of individuals from a community revealed a higher prevalence of diabetes in
Hispanics than in non-Hispanics.
9.17 Trends in post-neonatal mortality. Using vital statistics from
local health departments, researchers found post-neonatal mortality rates in Greenland had declined during the last 25 years. In
contrast, little progress has been made in decreasing neonatal mortality.
9.18 Community-Level Predictors of Pneumococcal Carriage and Resistance
in Young Children. Pneumococcal carriage and resistance vary markedly among
communities and are not fully explained by individual predictors. Community
risk factors may explain this variation. The authors of this
study geocoded addresses from a multicommunity sample of 710 Massachusetts children previously swabbed
for pneumococcal carriage. Using regression models, the authors evaluated
associations between census tract measures and pneumococcal carriage or
resistance. Living in census tracts with an average household size of more than
2.9 predicted carriage (odds ratio = 3.0, 95% confidence interval: 1.7, 5.5),
and living in socioeconomically disadvantaged census tracts conferred an
additional two- to threefold odds of carriage equal to attending child care.
Living in a census tract with a median household income of less than $35,000
predicted carriage among nonattendees (odds ratio =
2.7, 95% confidence interval: 1.7, 4.3). The predictive value of a low-income
census tract was interchangeable with any of several socioeconomic measures,
including poverty, unemployment, low educational attainment, and low owner
occupancy, in addition to high density of children and limited household
plumbing facilities. Furthermore, living in census tracts with low educational
attainment significantly predicted resistance (odds ratio = 4.0, 95% confidence
interval: 1.3, 12.7) and was interchangeable with a high density of children
(odds ratio = 3.5, 95% confidence interval: 1.0, 11.7). The two- to threefold
odds of pneumococcal carriage conferred by certain community characteristics suggest
that these measures may target communities for interventions to decrease
transmission. (American
Journal of Epidemiology; 2004; 159:645
- 654).
9.19 Pancreatic Cancer and Drinking Water and Dietary Sources of Nitrate
and Nitrite. N-Nitroso compounds, known animal
carcinogens, are formed endogenously from drinking water and dietary sources of
nitrate and nitrite. The authors conducted a a study
of pancreatic cancer in Iowa to determine whether increased consumption of
nitrate and nitrite from drinking water and dietary sources was associated with
risk. They linked detailed water source histories to nitrate measurements for Iowa community water
supplies. After exclusions for insufficient data, 1,244 controls and 189
pancreatic cancer cases were available for analysis. Among controls, the median
average nitrate level (1960�1987) was 1.27 (interquartile
range, 0.6�2.8) mg of nitrate nitrogen per liter of water. No association was
observed between pancreatic cancer risk and increasing quartiles of the community
water supplies� nitrate level. Increasing intake of dietary nitrite from animal
sources was associated with an elevated risk of pancreatic cancer among men and
women (highest quartile odds ratios = 2.3, 95% confidence interval: 1.1, 5.1,
for men and 3.2, 95% confidence interval: 1.6, 6.4, for women). In contrast,
dietary nitrate intake showed an inverse association with risk among women and
no association among men. This study suggests that long-term exposure to
drinking water nitrate at levels below the maximum contaminant level of nitrate
nitrogen (10 mg/liter) is not associated with pancreatic cancer; however, the
consumption of dietary nitrite from animal products may increase risk (American
Journal of Epidemiology, 2004; 159:693 - 701 -- full text).
