We conducted a systematic review to summarize the epidemiological evidence on the association between cigarette smoking, coffee drinking, and the risk of Parkinson's disease. Case-control and cohort studies that reported the relative risk of physician-confirmed Parkinson's disease by cigarette smoking or coffee drinking status were included. Study-specific log relative risks were weighted by the inverse of their variances to obtain a pooled relative risk and its 95% confidence interval (CI). Results for smoking were based on 44 case-control and 4 cohort studies, and for coffee 8 case-control and 5 cohort studies. Compared with never smokers, the relative risk of Parkinson's disease was 0.59 (95% CI, 0.54-0.63) for ever smokers, 0.80 (95% CI, 0.69-0.93) for past smokers, and 0.39 (95% CI, 0.32-0.47) for current smokers. The relative risk per 10 additional pack-years was 0.84 (95% CI, 0.81-0.88) in case-control studies and 0.78 (95% CI, 0.73-0.84) in cohort studies. Compared with non-coffee drinkers, relative risk of Parkinson's disease was 0.69 (95% CI, 0.59-0.80) for coffee drinkers. The relative risk per three additional cups of coffee per day was 0.75 (95% CI, 0.64-0.86) in case-control studies and 0.68 (95% CI, 0.46-1.00) in cohort studies. This meta-analysis shows that there is strong epidemiological evidence that smokers and coffee drinkers have a lower risk of Parkinson's disease. Further research is required on the biological mechanisms underlying this potentially protective effect.

Objective To explore the association between migraine and risk of ischaemic stroke. Design Systematic review and meta-analysis.

The identification of heterogeneity in effects between studies is a key issue in meta-analyses of observational studies, since it is critical for determining whether it is appropriate to pool the individual results into one summary measure. The result of a hypothesis test is often used as the decision criterion. In this paper, the authors use a large simulation study patterned from the key features of five published epidemiologic meta-analyses to investigate the type I error and statistical power of five previously proposed asymptotic homogeneity tests, a parametric bootstrap version of each of the tests, and tau2-bootstrap, a test proposed by the authors. The results show that the asymptotic DerSimonian and Laird Q statistic and the bootstrap versions of the other tests give the correct type I error under the null hypothesis but that all of the tests considered have low statistical power, especially when the number of studies included in the meta-analysis is small (<20). From the point of view of validity, power, and computational ease, the Q statistic is clearly the best choice. The authors found that the performance of all of the tests considered did not depend appreciably upon the value of the pooled odds ratio, both for size and for power. Because tests for heterogeneity will often be underpowered, random effects models can be used routinely, and heterogeneity can be quantified by means of R(I), the proportion of the total variance of the pooled effect measure due to between-study variance, and CV(B), the between-study coefficient of variation.

Considerable controversy exists about the role of education in the risk of dementia. Individual studies have not been conclusive so far. To examine the hypothesis that lower education is associated with a higher risk of dementia, we carried out a meta-analysis. Observational studies published as of October 2005 that examined the association between education and risk of dementia were systematically reviewed. Relative risks (RRs) and odds ratios were extracted from cohort and case-control studies. We first compared the risk of dementia in subjects with high level of education with the risk of dementia in those with low educational level. In a subsequent analysis, we compared the risk of persons with high education with the risk of subjects with education level other than high (medium, low). We weighted log RRs for cohort studies or odds ratios by the inverse of their variances. Nineteen studies were included in our meta-analysis (13 cohort and 6 case-control studies). RRs for low versus high education level were: Alzheimer’s disease (AD) 1.80 (95% CI: 1.43–2.27); non-AD dementias, 1.32 (95% CI: 0.92–1.88), and all dementias 1.59 (95% CI: 1.26–2.01). For low and medium versus high education level, the RRs were: AD 1.44 (95% CI: 1.24–1.67); non-AD 1.23 (95% CI: 0.94–1.61), and all dementias 1.33 (95% CI: 1.15–1.54). These results confirm that low education may be a risk factor for dementia, especially for AD.

Globally, the increase in the risk of fractures among psychotropic drug users is moderate. Further research is needed, especially to examine high-risk populations and newer medications. Future studies should be prospective and emphasise control of confounding bias.