Study designs case-control studies

In the case of long-term use of tetracyclines for acne, at least 7 cases of contraceptive failure have been reported. Nevertheless, in statistical terms the only well-designed case-controlled study in dermatological practice indicated that the incidence of contraceptive failure due to this interaction could not be distinguished from the general and recognised failure rate of oral contraceptives. ... 

In this review we concentrate on individual-based studies. In case-control studies, subjects with already existing disease are retrospectively compared with control persons with respect to previous exposure. In cohort studies, individual exposure is measured at baseline and the cohort is followed over time for newly diagnosed outcome variables. Both case-control and cohort studies estimate the (relative) risk associated with exposure. Intervention studies utilize an experimental design and are mostly double blind trials with a defined treatment or placebo assigned randomly. Results from intervention studies, followed by cohort studies, make the greatest contributions to obtaining evidence of a causal relationship. Case-control studies are liable to particular bias, which give their results less credibility. 

Strength of recommendation A based on a meta-analysis or at least one randomized controlled trial. Strength of recommendation B based on at least one well-designed study, including case control and comparative studies. Strength of recommendation C based on expert reports or opinion (levels of evidence and strength of recommendation. Oxford (UK) Centre for Evidence-Based Medicine. Available at http //www.cebm.net/levels of evidence.asp (accessed December 8,2008). 

Although earlier work had showed a positive association between coffee consumption and pancreatic cancer rates across countries,2 it was the much-publicized case-control study of MacMahon et al.3 in 1981 that attracted widespread attention to the question of a possible link. In that study, which was designed primarily to investigate the role of smoking and alcohol in pancreatic cancer, 369 pancreatic cancer patients prior to diagnosis and 644 hospital controls reported their typical daily coffee and tea consumption. Unexpectedly, the authors found a significantly increased risk of pancreatic cancer associated with coffee consumption (overall rela-... 

Bradbury et al. (2004), however, recently reanalyzed the relation between tamoxifen and cataracts and described it as a null association, They used a nested, matched, case-control study design and data collected in the General Practice Research Database. They identified all women 30-79 years old who were diagnosed with breast cancer and treated with tamoxifen within 6 months, or with bladder cancer, colorectal cancer, or nonmelanoma skin cancer between January 1991 and December i999. From this population they identified all newly diagnosed cases of cataract and matched four female controls to each case on age, index date, and study entry data. They assessed the risk of cataracts for current, past, and sometime users of tamoxifen... 

Epidemiological studies have different strengths and weaknesses associated with their design (Table 4) shows some of the strengths and weaknesses of the two main methods of prospective cohort studies and retrospective case-control studies. 

Breslow, N.E. Day, N.E. (1980) Statistical Methods in Cancer Research, Vol. 1, The Analysis of Case-Control Studies (lARC Scientific Publications No. 32), Lyon, ARCPress Breslow, N.E. Day, N.E. (1987) Statistical Methods in Cancer Research, Vol. 2, The Design and Analysis of Cohort Studies (lARC Seientific Publications No. 82), Lyon, IP RCPress Cohen, S.M. Ellwein, L.B. (1990) Cell proliferation in carcinogenesis. Science, 249, 1007-1011... 

High-dose lead exposure is a recognized risk factor for stillbirth or spontaneous abortion. Epidemiologic studies of the impact of low-level lead exposure on reproductive outcome such as low birth weight, preterm delivery, or spontaneous abortion have yielded mixed results. However, a well-designed nested case-control study recently detected an odds ratio for spontaneous abortion of 1.8 (95% Cl 1.1-3.1) for every 5 / g/dL increase in maternal blood lead across an approximate range of 5-20 g/dL (Boija-Aburto et al, 1999). In males, blood lead concentrations in excess of 40 / g/dL have been associated with diminished or aberrant sperm production. 

Case-control studies have been designed to determine if 2,3,7,8-TCDD exposure results in increased risks for site-specific cancers. Case-control studies have found significant increases in the risk of soft-tissue sarcomas in Swedish agricultural, forestry, and horticultural workers (Eriksson et al. 1981, 1990 Hardell and Eriksson 1988 Hardell and Sandstrom 1979), workers involved in manufacturing and application of phenoxy herbicides (Kogevinas et al. 1995), and New Zealand farmers (Smith et al. 1984a). In the Eriksson et al. (1990) study, the risk ratio of soft-tissue sarcoma was 1.80 (95% 0=1.02-3.18) in subjects exposed to phenoxyacetic acid herbicides and/or chlorophenols. In subjects exposed to phenoxyacetic acid herbicides only or chlorophenols only, the risk ratios were 1.34 (95% 0=0.7-2.56) and 5.25 (95%... 

The cohort study offers two additional strategies for analysis — case-cohort and nested case-control study designs the case-cohort comprises all individuals in the cohort with a particular disease and a random sample of unaffected individuals from the cohort the nested case-control design includes cohort members with a particular disease (cases) and unaffected individuals (controls) matched on relevant study covariates ... 

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