The seriation method works because object styles change over time; they always have and always will. A good example of change in artifact type is the development of hand-held PDAs from those first enormous cell phones. Beam me up, Scottie! As an example of how change through time works, consider the different music recording methods that were used in the 20th century. One early recording method consisted of large plastic disks which could only be played on a huge device called a gramophone. The gramophone dragged a needle in a spiral groove at a rate of 78 revolutions per minute (rpm). The gramophone sat in your parlor and certainly couldn't be carried along with you and your earbobs. Thank goodness for mp3s.
When 78 rpm records first appeared on the market, they were very rare. When they became popularly available, you could find them everywhere; but then the technology changed and they became rare again. That's change over time.
Archaeologists investigate trash, not shop window displays, so we measure things when they are discarded; in this example we're going to use junkyards. Archaeologically, you would expect no 78s to be found in a junkyard that was closed before 78s were invented. There might be a small number of them (or fragments of them) in the junkyard which stopped taking junk during the first years 78s were invented. You would expect a large number in one closed when 78s were popular; and a small number again after 78s were replaced by a different technology. You might find a small number of 78s for a long period after they were pretty much done. Archaeologists call this kind of behavior "curation" -- people then, just like today, like to hang on to old things. But you would never have any 78s in junkyards closed before they were invented. The same is true for 45s, and 8-tracks, and cassette tapes, and LPs, and CDs, and DVDs, and mp3 players (and really, any kind of artifact).
Sources and Further Information
- Timing is Everything: A Short Course in Archaeological Dating Techniques
- Sampling in Archaeology
See the bibliography for a list of sources and further reading.