Data were extracted from respected third-party sources, both electronic and print, that in the judgment of the compilers are the most current and accurate sources to which they have access. For many countries, data were either not available (indicated in the charts as NA) or sporadic. Also, for many countries of the world, the data were not as current as the compilers would have liked. Printed sources were several years old and many websites were suspect. It was felt, however, that a fairly recent figure was better than none at all. In the case of "World Guide to Libraries," the data for total number of volumes was tabulated using the CD-ROM version since they were not published as country totals. The compilers had to determine which of the sometimes several possible sources was the most reliable in terms of accuracy and currency. Seldom did the data from two or more different sources coincide for the same year.
In those few instances when the date of the source data was unclear or unknown, the date of compilation or of publication was used.