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This is the first paper in a series dedicated to the study of theemission-line and continuum properties of narrow line Seyfert 1 galaxies(NLS1s). We carried out a systematic search for NLS1s from objects assigned as``QSOs'' or ``galaxies'' in the spectroscopic sample of the Sloan Digital SkySurvey Data Release 3 (SDSS DR3) by a careful modeling of their emission linesand continua. The result is a uniform sample comprising $\sim 2~000$ NLS1. Thissample dramatically increases the number of known NLS1 by a factor of $\sim 10$over previous compilations. This paper presents the parameters of the prominentemission lines and continua, which were measured accurately with typicaluncertainties $<10%$. Taking advantage of such an unprecedented large anduniform sample with accurately measured spectral parameters, we carried outvarious statistical analysis, some of which were only possible for the firsttime (abbreviated).Comment: 30 figures available in "jpg" format, accepted for publication in the  Astrophysical Journal Supplement

A Comprehensive Study of 2000 Narrow Line Seyfert 1 Galaxies from the Sloan Digital Sky Survey. I. The Sample