Mean Ceramic Dates

            For the Tommy Site assemblage, weighted mean ceramic dates and ranges were calculated for proveniences with ceramic types having less than a 300-year temporal span.  Mean ceramic dates were calculated using the general formula proposed by South (1977) and have been implemented in analysis of Anasazi ceramic assemblages by numerous researchers (Goetze and Mills 1993; Goff and Reed 1998; Hays-Gilpin et al. 1999; Reed and Hensler 1999).   In this study, proveniences were dated ceramically by calculating mean dates using the types and date ranges identified below.  Mean ceramic dates were calculated using Excel^® 97 for proveniences containing temporally sensitive ceramic types by a four-step calculation process.  First, for a specific provenience (PD), types with over a 300 year temporal span were eliminated from the date calculation and only ceramic types with a narrow (<300 year) range were used in the mean date calculation.  Second, each remaining type was weighted by calculating a type percentage.  The type percentage was calculated by dividing the number of items representing each type by the total number of items remaining in the provenience (after eliminating sherds with over a 300 year temporal span).  Third, the early dates (see table below) for each type were multiplied by the type percentage and then all early weighted dates were added together to produce a single weighted early date for the provenience.  The same calculation was repeated for the late dates for each type in the provenience.  Finally, the mean for the weighed early and late dates was calculated by adding the early date and late date together and then dividing by two. 

            Once the mean date for a provenience was calculated, a range (see Reed and Hensler 1999) was calculated by first subtracting the early weighted date from the late weighted date and dividing that number by four to obtain a range value.  Then, subtract the range value from the mean date and add the range value to the mean date to get the mean date range.   Reed and Hensler (1999) have successfully used this method of calculating mean ceramic date ranges for the N33 Road Project in northeastern Arizona.  Their calculated ceramic date ranges for sites excavated along the N33 Road compared favorably with absolute chronometric dates (tree ring and archaeomagnetic dates) obtained from the sites.  In fact, many of the mean ceramic date ranges fall within 10 years of the absolute chronometric dates. 

Click here for a graphic summary of the ceramic mean date ranges (by sherd count and sherd weight) for each stratigraphic level of the midden represented by Unit 142N, 102E.

Table of Ceramic Types and Date Ranges of Manufacture Used to Calculate Mean Dates and Ranges.