The AAVSO holds a data archive of over 20 million variable star observations collected by thousands of observers over more than a century. This archive is known as the AAVSO International Database, or AID. These data come from a number of sources, including:
Data currently in the AID are exclusively optical and NIR photometry; the AID does not include other multiwavelength photometry or spectroscopy, and does not include time-of-maximum or -minimum data. In all cases, these data were either submitted directly to the AAVSO by the observer or were collected by the AAVSO or its volunteers from published or unpublished sources. The majority of the data were submitted to the AAVSO by observers in the amateur astronomical community; prior to the year 2000, the majority of the observations are visual magnitude estimates made by comparing the apparent brightness of a star to a previously calibrated comparison star sequence within the field. The great majority of our long-term visual light curves begin between 1900 and 1950, with a small number stars having longer-term light curves collected from literature sources.