When I teach people how to conduct a proper search of the patent literature, I always find it challenging to convince them that they need to include older patents in their searches. Most people, especially undergraduate students, are content to do keyword searches that will retrieve only patents from the mid-1970s forward. They seem to believe that older patents couldn't possibly be relevant to whatever technology they're interested in. Of course, this is not the case: there are many technologies that can trace their origins back to the mid-19th century (or even earlier).
I decided recently that I needed some hard data to back up my arguments, so I looked at the cited references in recently issued patents in selected USPC classes and subclasses. References are cited by applicants and patent examiners based on their relevance to the patented invention. Of course, this includes patents back to 1790. For this exercise I chose bicycles (280/2*), stirling engines (60/517), wind turbines (Class 416), fuel cells (Class 429), hydrocarbons (Class 580) and surgical instruments (Class 606). I retrieved the 20 most recent patents in each class/subclass and compiled the references to US patents in a spreadsheet. The results were very interesting.
In four of the five classes/subclasses, the total number of references ranged from 247 to 297; the median number of references ranged from 6 in fuel cells (Class 429) to 11.5 in wind turbines (Class 416). Surgical instruments (Class 606) had 1110 references, almost four times the average in the other four categories, with a median of 45 references per patent. The earliest references ranged from 1867 in surgical instruments to 1966 in fuel cells. Fuel cells had the fewest pre-1976 references (1 or 0.44 percent). But approximately 10 percent of the references in stirling engines, hydrocarbons and bicycles were earlier than 1976. Surgical instruments had 5 percent and wind turbines had 7 percent.
This data illustrates the importance of pre-1976 patents in the technological record and makes a strong argument for searching by patent classification. With the exception of fuel cells, all categories had a substantial number of references prior to 1976 which would not be retrieved by keyword searches.