If the data are stored in the computer with files for each year, it is possible to undertake analyses of these types. In Ontario, six species of birds that had bred in Ontario were not found during the atlas period. This almost certainly means that they are now no longer present as native breeders in this area. In some cases atlas data have been collected continually. The maps for moat species have three types of records, those before 1900, those between 1900 and 1959, and those between 1969 and 1976.
Historical information is prone to bias due to the under collection of data in some areas.
However, if used judiciously, it can still be used to show changes over time. This is most certainly the case for those species that were once found in an area but were found more sparingly during more recent data collections. Determination of Rarity Atlas data show the geographical distribution of a species. This information can be used to give a rough idea of the relative rarity of an organism. Certainly, if a species is found in a majority of the squares in a province it cannot be considered rare.