Finally, you move the bars vertically until each artifact percentage bar group lines up together in what is known as a "battleship curve", narrow at both ends, when the media shows up less frequently in the deposits, and fatter in the middle, when it occupies the largest percentage of the junkyards.
Notice that there is overlap--the change isn't an abrupt one, so that the previous technology isn't instantly replaced by the next. Because of the stepped replacement, the bars can only be lined up in one of two ways: with C at the top and F at the bottom, or vertically flipped, with F at the top and C at the bottom.
Since we know the oldest format, we can say which end of the battleship curves is the starting point. Here's a reminder of what the colored bars represent, from left to right.
- 78 rpm
- 33 1/3 rpm
- 45 rpm
- 8 Track
- Cassette
- CD Rom
- DVD
In this example, then, Junkyard C was likely the first opened, because it has the largest quantity of the oldest artifact, and lesser amounts of the others; and Junkyard F is likely the most recent, because it has none of the oldest type of artifact, and a preponderance of the more modern types. What the data doesn't provide is absolute dates, or length of use, or any temporal data other than the relative age of use: but it does allow you to make inferences about the relative chronologies of the junkyards.
Why is Seriation Important?
Seriation, with some modifications, is still in use today. The technique is now run by computers using an incidence matrix and then running repeated permutations on the matrix until it falls out in the patterns shown above. However, absolute dating techniques have made seriation a minor analytical tool today. But seriation is more than a footnote in the history of archaeology.
By inventing the seriation technique, Petrie's contribution to chronology was an important step forward in archaeological science. Completed long before computers and absolute dating techniques such as radiocarbon dating were invented, seriation was one of the earliest applications of statistics to questions about archaeological data. Petrie's analyses showed that it is possible to recover otherwise "unobservable hominid behavior patterns from indirect traces in bad samples", as David Clarke would observe some 75 years later.
Sources and Further Information
Timing is Everything: A Short Course in Dating Techniques
McCafferty G. 2008. Seriation. In: Deborah MP, editor. Encyclopedia of Archaeology. New York: Academic Press. p 1976-1978.
Graham I, Galloway P, and Scollar I. 1976. Model studies in computer seriation. Journal of Archaeological Science 3(1):1-30.
Liiv I. 2010. Seriation and matrix reordering methods: An historical overview. Statistical Analysis and Data Mining 3(2):70-91.
O’Brien MJ and Lyman LR 1999. Seriation, Stratigraphy, and Index Fossils: The Backbone of Archaeological Dating. New York: Kluwer Academic/Plenum Publishers.
Rowe JH. 1961. Stratigraphy and seriation. American Antiquity 26(3):324-330.
