It is difficult and costly venture to produce a really authoritative encyclopaedia. They get out of date in a course of time due to dynamic nature of knowledge. Hence, they have to be updated and revised at some intervals. They can be revised using different methods.
Publication of Annual Supplementary Volume: Some publishers publish a supplement to encyclopaedia every year updating the facts that have undergone change and covering important events of that particular year. One has to lake sure whether the arrangement of the supplement is similar to that of the basic set.
Periodical Revision : There are some publishers who produced the revised and reset edition of the entire set of an encyclopaedia after a specific interval of time. This enables them to publish a revised, new edition updating the knowledge.
Continuous Revision Policy Some publishers employ permanent editorial staff who carry on systematic revision of incorporating changes in the contents of the articles. The articles covering the continents, states, cities, towns, villages that undergo changes various other reasons are regularly updated. Thus, the changes in such articles due to new developments are incorporated whereas the other articles remain stable.
The reference librarian should be cautious while examining the so-called new editions of the set. The publisher may claim to have continuous revision policy but there is a big gap between the actual revisions carried out and the claims made of updating. It is often found that changes are made in some evident science and technological fields but the other subjects are completely neglected.