This article requires a subscription to view the full text. If you have a subscription you may use the login form below to view the article. Access to this article can also be purchased.
The classical correlational investigation is now commonly referred to as an ecologic study. Multiple sclerosis (MS) is particularly suited for this analytical approach. Repeated investigations have confirmed not only its marked geographic heterogeneity at the global level and predominance in Caucasians,1-4 but also remarkable geographic variations within individual countries5-12 and even within smaller sub-areas.13-15 Such findings make ecologic comparisons useful and provide excellent opportunities to a search for consistent associations as an important criterion for causality.16
The place of the ecologic study within the armamentarium of epidemiologic designs is somewhat controversial. Most modern textbooks17-20 include it with the descriptive methods. This may be correct operationally, but when the purpose of the study is considered, the ecologic study is analytical research,21 as the purpose is to venture beyond pure description by systematically considering the environment as a source of putative risk factors.
The utility of the ecologic study is also a matter of debate. Most authors agree that it is a valuable tool for the generation and early support of hypotheses, but several have argued that it permits only limited conclusions because of well-recognized shortcomings (see below).17-27 Yet others28 attribute to the ecologic approach a high value in the process of causal inference in the face of the established limitations of case-control studies and the frequent impracticality of cohort studies. The low rank commonly assigned to the ecologic study in the process of causal inference refers to the single investigation. When an extended and systematic search is conducted for an association between the disease in question and a defined exposure by multiple investigations covering independent areas and populations, and when, in addition, these include smaller areas giving less rise to confounding, considerable power can be attained in the ability to test epidemiologic criteria …
AAN Members
We have changed the login procedure to improve access between AAN.com and the Neurology journals. If you are experiencing issues, please log out of AAN.com and clear history and cookies. (For instructions by browser, please click the instruction pages below). After clearing, choose preferred Journal and select login for AAN Members. You will be redirected to a login page where you can log in with your AAN ID number and password. When you are returned to the Journal, your name should appear at the top right of the page.
AAN Non-Member Subscribers
Purchase access
Letters: Rapid online correspondence
REQUIREMENTS
You must ensure that your Disclosures have been updated within the previous six months. Please go to our Submission Site to add or update your Disclosure information.
Your co-authors must send a completed Publishing Agreement Form to Neurology Staff (not necessary for the lead/corresponding author as the form below will suffice) before you upload your comment.
If you are responding to a comment that was written about an article you originally authored:
You (and co-authors) do not need to fill out forms or check disclosures as author forms are still valid
and apply to letter.
Submission specifications:
- Submissions must be < 200 words with < 5 references. Reference 1 must be the article on which you are commenting.
- Submissions should not have more than 5 authors. (Exception: original author replies can include all original authors of the article)
- Submit only on articles published within 6 months of issue date.
- Do not be redundant. Read any comments already posted on the article prior to submission.
- Submitted comments are subject to editing and editor review prior to posting.
