Various classification systems exist and most have been designed so that each technical aspect of an invention to which a patent document relates can be used for classifying as a whole. These are too general to meet the needs and demands of specific technology areas. Is it efficient to search invention only using keywords? How often are patent classification codes used for patent search?
I would not try to use one or the other. You might miss something critical. As Mikk Putk stated, start with a keyword search, find a variety of classifications referenced in these patents, and base your classification search from these classifications.
I agree with the writers above. Keywords work well only if you can think up the words that match those used by the people who drafted the patents. I always use keywords to find close patents and then search classifications to catch the patents that contain words that I did not imagine. Of course, patent classification is a human activity, so there will always be errors and omissions. Keyword searches help you catch those.
I also agree with the writers above. Don't start a search using a classification code, you might limit your search too much, suppresing relevant patents. Try to identify most relevant keywords (and their synonyms; also use truncation to cover different endings, singular/plural etc.) and create a "document pool" by combining them (Boolean operators, proximity operators). From that "pool" you can start refining the search with more specific keywords, classification codes etc. (I tend to use IPC/CPC codes late in a search). Don't use one single search string only. From that "pool" you can drill down several times fosussing on different aspects of the search. In the end you combine the results of the single searches, eliminating duplicates. The resulting set of answers (hopefully not too big) you'll have to check one-by-one opening the patent and going through the claims.
Classification codes are powerful tools, but you have to be aware of what you might miss. In my opinion they are also more powerful when dealing with "main topics" rather than dealing with minor aspects of a patent (as Sandra said: "classification is a human activity").
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