Periodontitis is one of the most prevalent oral diseases. In this study, we probed the nationwide registered database to assess the time trends of prevalence of periodontitis in Taiwan.A retrospective study was conducted to analyze the registered database compiled by the National Health Insurance provided by the Department of Health, Taiwan, from 1997 to December 2013.We found that the prevalen...

We derive an expression for the lower bound of the phase-space beam quality factor M(2) of an ultrashort pulse. We show that the condition M(2)>or=1 does not, in general, hold for such pulsed beams. Rather, the minimum value of M(2) depends on the pulse spectrum. We also show that M(2) attains minimum only for pulsed beams whose spot size varies with frequency as omega(-1/2), and that the radia...

The role and limitations of retrospective investigations of factors possibly associated with the occurrence of a disease are discussed and their relationship to forward-type studies emphasized. Examples of situations in which misleading associations could arise through the use of inappropriate control groups are presented. The possibility of misleading associations may be minimized by controlli...