Weibull Estimation on Use Life Distribution in Experimental Spear-Point Data

ABSTRACT

Archaeologists routinely give meaning to the size and composition of stone tool assemblages in terms of activity and cultural affinity, but these assemblage properties also are affected by use life or longevity. All else being equal, longer use reduces discard rate and therefore assemblage size; use life difference between classes affects composition separately from their notes of use. Use life is important to assemblage Interpretation. It can be expressed both as a mean value across specimens in a class and also as a distribution of actual use-life value among them, which affects the assemblage formation independently of the mean. Thus we need to know both mean and distribution of stone tool use lives. The Weibull model estimates both quantities, and its governing theory distinguishes between distributions produced by accident or chance and by cumulative attrition. Experimental data from the replication and use of stone-tipped projectiles yiled mean use lives of about 3–4 firings, but distributions that vary with point size, design and context of use. Weibull analysis suggests that weight and tip angle of points contribute to longevity and that heavier and more obtuse points generally survive longer because they resist chance fracture. Failure shows an exponential distribution—owing to chance—in smaller points, but a Weibull distribution—a combination of chance and attrition—in heavier ones.