A method of calculating the date of American Colonial assemblages based on the variation in hole diameters in clay pipe stems. J.C. Harrington first drew attention to the fact that there is a general reduction in hole size from 1620-1800. Lewis Binford then developed a regression equation, thus: [ y = 1931.85 - 38.26x ] where y is the mean date for the group and x is the mean pipe-stem diameter for the sample. ("A New Method of Calculating Dates from Kaolin Pipe Stem Samples" Lewis R. Binford) The formula works well for the period 1680-1760 but fails to produce satisfactory results for post-1780 assemblages.