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The overarching goal of this database is to serve as a comprehensive, unbiased, centralized, publicly available and regularly updated collection of epidemiologic reports that evaluate environmental (i.e., non-genetic) risk factors for Alzheimer disease (AD) in well-defined study cohorts. Eligible publications are identified through contact with each cohort study supplemented by a systematic review of the scientific literature databases. To ensure the highest degree of objectivity regarding the posted information, only studies published or in press in peer-reviewed journals available in English are considered for inclusion into the database. In addition, the results that we present must be adjusted, at minimum, for age and sex (as appropriate). Abstracts presented at scientific meetings or findings reported in non-peer-reviewed publications are not considered for inclusion. The database is currently in development, so data are available only for a limited selection of risk factors, with additional risk factors to be added in the coming year.
While we have made every possible effort to correctly represent the data of all studies fulfilling the above criteria, we cannot exclude the possibility that some studies are cited incorrectly or are erroneously excluded. In any case, we are not able to make any warranty, either expressed or implied, with respect to the functioning and accuracy of this database. No responsibility is assumed by the authors and curators.
A Note on the Differences Between Observational Studies and Clinical Trials
The reports in this database describe primarily observational studies, meaning studies in which the investigators had no control over the exposure status of the study participants. This design stands in contrast to a clinical trial, in which investigators assign (typically randomly) participants to receive different levels of the exposure, i.e., a medication or intervention aimed at treating or preventing disease. The bulk of clinical trials in the Alzheimer disease field are treatment trials, but the trials relevant to this database are prevention trials, which are fewer in number.
When properly designed, conducted, and analyzed, both observational studies and prevention trials can produce valid findings. A key potential limitation of observational studies, however, is that other factors could be related to both the exposure and outcome, leading to a spurious association between the two (i.e., “confounding”). Prevention trials are designed to minimize confounding, and are thus in theory superior to observational studies, but they have their own limitations, and are not feasible for many exposures (e.g., sex, education, head injury, or long-term dietary or exercise habits). For some exposures, prevention trials and observational studies have given discrepant results. Some of these discrepancies may be due to confounding, and thus the clinical trial data may be more credible. Others may have occurred because the observational studies and prevention trials asked different questions—by selecting a different population for study or studying a different timing, duration, or magnitude of exposure. Overall, the two types of studies are probably complementary. In any case, any discrepancies between observational studies and prevention trials are explicitly noted on the appropriate Risk Factor Overview page, and discussed in detail in the associated documentation; we also note differences in any relevant treatment trials. We advise users of this database to interpret the findings from the individual reports and meta-analyses with caution.
Please note that while the meta-analyses presented in this database take into account between-study heterogeneity, they do not consider potential biases in the published results, including but not limited to confounding factors, differential selection or survival, and publication bias, i.e., that the outcome/significance of any particular study directly influences its probability of publication. Thus, the risk factor-specific summary effect estimates presented in the meta-analysis section may be biased either upward or downward, depending on the quality of the studies for the particular risk factor.
To report an error in the data or to submit general concerns, comments, and suggestions about the database, please
. We encourage authors of original AD epidemiologic association studies published (or in press) at a peer-reviewed scientific journal to send us their data.