When examining an encyclopedia with an eye to purchase, consider these points:

* How does it look? Are there good graphics to help explain some of the more technical subjects--like nuclear reactors? Is the type attractive and legible? Is color used in maps, charts and photographs?

Appearance often is important, especially to younger users who, one hopes, will find pleasure just reading the encyclopedia.

* How easy is the set to use? If children will be primary users, then the format should be fairly easy to follow. An index volume is virtually essential. Outlines within various entries also will help guide a neophyte through a formidable subject.

* What sort of reference tools does the encyclopedia offer for someone doing research? Are bibliographies included after most of the longer entries? How thoroughly are subjects cross-referenced? Encyclopedia Buying Guide includes a handy chart listing, among other things, the number of cross-references included in various sets.

* How readable is the encyclopedia? Is it suitable to your children's reading ability? How long will it be suitable? Most encyclopedias are written for specific audiences.

* Finally, and perhaps most difficult to assess, what is the reliability of the encyclopedia? Reading entries at random may give you some sense of how accurate the work is. You can also check out various contributors in "Who's Who."

A more important consideration is timeliness. Given the changes in science and technology, not to mention politics, encyclopedias are hard-pressed to keep up. Although some claim "continuous revision," only about one-third of any given encyclopedia's entries are revised annually, so even though an encyclopedia may carry a recent copyright date, its entries may not all be up-to-date.

As for used encyclopedias: Beware of buying any that are 5 years old or older. Don't consider any that are more than 10 years old.