Now there's a phrase you probably have never heard before. Any guesses what it means? No worries--I'm about to tell you. Federated searching is when you use a search tool to search multiple databases at once. Say, searching Academic Search Premier and ERIC at the same time. On the surface, this is nothing but good news. I mean, you can do one search and get all your results at the same time, instead of having to run two searches. If you were planning on searching three or four databases, it just gets better, right?
Or at least, wrong some of the time. In an ideal world--one with lots of butterflies, unicorns and Puff the Magic Dragon--you could search all databases at once and find all the information you're looking for. But we don't live in that world (sorry, unicorn-lovers out there). We live in a world where databases have different capabilities and options.
To explain this, allow me to turn to math for a moment. Remember that thing called the "lowest common denominator"? It's the least common multiple shared between a number of fractions. When adding or subtracting fractions, you get everything into the essential same format, then run the process. That's what happens with federated searching. All those bells and whistles and tips and tricks that individual databases have are steamrolled out in the rush to make them all work together. Does the database have a cool thesaurus that suggests alternate search terms? Gone. Can it separate out peer reviewed journals? See ya. Narrow by subject? Buh-bye.
Please note that this sometimes isn't the case. If all databases being searched all share the same features, then nothing need be lost in the transition. And thankfully, database providers (and federated search providers) are working on this problem. I haven't checked recently, but I imagine it's much less of an issue today than when I was researching it in depth a few years ago, and I expect it will continue to improve as the years go by.
But now you know what's at stake when you're using a federated search, so please: search responsibly. :-)