All EDC systems have some form of rules facility. The rules are typically designed to check data. If the check fails, then typically a query is produced.
Web based EDC systems typically run edit checks when a page is submitted.
Prior to the release of web based systems, it was typical to check the data as soon as it was entered - between fields.
With Web 2.0 technologies, it may prove possible to routinely run checks as soon as data is entered - prior to page submit.
So - a question - which approach is best?