More Experiments in Intensive Pedestrian Survey
More guest-posting brilliance from our esteemed guest blogger, David Pettegrew, the co-director the Pyla-Koutsopetria Archaeological Project and the 2010 Cyprus Research Fund speaker. Be sure to check out his posts on Tuesday, Wednesday, and Thursday.
Last Thursday, we introduced the survey experiment that PKAP conducted in June 2010 to assess the relationship between the number of artifacts that we see when we walk across a survey unit and the number of artifacts actually on the ground. In other words, we wanted to assess how effective our survey methods are in actually assessing what was on the ground. On Thursday, we compared the artifact densities detected by the project’s untrained student fieldwalkers to those counted by trained senior staff members. Today we will discuss the second phase in our 2010 experiment, an assessment of the total population of all artifacts on the surface of select subunits. This part of the experiment was designed to give us a total count of all surface artifacts that can be compared with the artifact counts reported in yesterday’s discussion.
We began by selecting four 10 x 10 m subunits based on the densities of the 10 x 10 m artifact densities counted by the experienced senior staff members. As with past experiments (published in the RDAC 2007), we selected our 4 subunits to represent the range of density variation: the lowest density quartile (G15), highest density quartile (G9), and two middle quartiles (G1 and G6). Each total subunit was 10 x 10 m, representing 1/16 (6.25%) of the 1,600 sq m survey unit.
To vacuum a high-density unit, you really have to spend a lot of time picking individual artifacts off the ground. For each of our units, students Andrew, Zane, Valerie, and Luke, and I walked very slowly in adjacent passes across each selected square gathering together in 1 or 2 corners of the unit all the artifacts present. An initial pass was never enough for we observed how many artifacts we missed initially. Usually two additional passes were necessary to vacuum the surface completely, and each pass involved either crawling on hands and knees, or bending so that you had a closer view of the ground. I have to admit that my back and neck got sore after a while of this.
The results of this “total collection”, shown below, are interesting to compare with the “pedestrian survey counts” discussed yesterday. You have to keep in mind with the comparison that the pedestrian counts represent a 20% sample of each subunit while the total collection counts represent a 100% sample. You have to multiply the pedestrian count by a factor of 5 to estimate the “total putative count” (i.e., an estimation of what the total count would be for 100% of the unit) for the pedestrian-walked unit.
The first outlined set of grid units below shows the total counts from each of the total collection units.
The second set of grids compares the total collection counts with the pedestrian survey counts in parentheses (multiplied by 5 to create the 100% putative sample).
The third shows the factor difference between these two types of counts.
Here is where it gets even more interesting. We can estimate that the 940 artifacts experienced fieldwalkers counted through pedestrian survey across the entire unit (i.e., the pedestrian counts from 4 walker swaths) would produce a putative pedestrian survey count (factoring for the 20% sample) of 4,700 artifacts. In other words, had we walked 100% of the unit, we would have counted about 4,700 artifacts. Now, if total collection (vacuuming) produces on average 2.96 times the number of artifacts as pedestrian survey, we can estimate that there were 13,212 artifacts actually on the surface of the ground. To provide some perspective, we collected and brought back to the museum 8,788 total artifacts from the 252 grid squares of Koutsopetria and 19,657 total artifacts from our survey of the entire Pyla-Koutsopetria area. A single survey unit at Koutsopetria totally collected would produce 1.5 times the number of artifacts sampled from all 252 grid squares at Koutsopetria and .67 of the total artifacts sampled across the entire Pyla area. If we were to apply the same multipliers to all 252 forty x forty meter grid squares, i.e., the main part of the site of Koutsopetria, the total artifact count of 19,182 would produce a putative total count of 95,910. Our estimated total population of artifacts (based on the 2.96 factor) is at least 284,894 (and in reality, poor visibility in many units often limited our sample to 50% of the ground). This is *why* sampling is important!
As for TIME, total collection requires a huge commitment. Although we (for clarification here, “we” means David – Bill) initially considered surveying all 16 subunits, i.e., an entire 40 x 40 m unit, this proved unrealistic given the time it took for 5 individuals to vacuum a single subunit: 1.5 hours each for G1 and G6, 2 hours for G9, and 1 hour for G15. Using the total time it took to hoover 25% of the grid square (6 hours) as an index for hoovering this unit, we estimate that 5 individuals could hoover a high-density 40 x 40 m unit in about 24 work hours or well over 100 work hours! If the typical survey work day is 6 hours long (say, 6AM-noon), it would require 4 full days of a team collecting artifacts from the surface. Truly this would be an incredibly time intensive task! By contrast, sampling 20% of the unit through pedestrian survey takes about 20-30 minutes. In this perspective, total collection requires 72 times more time than pedestrian survey collection!
One final comparative result is interesting to note here. The “other” category increases dramatically through total collection, including numerous pieces of ancient glass (9), lithic stone artifacts (7), shells (24), slabs (13), gypsum (141), ceramic bricks (2), stone vessel (1), marble revetment (3), and a ceramic tessera or gaming piece. Although total collection was time intensive, this sort of qualitative information is quite useful in filling out our picture of the overall survey unit and indicates something of the functional variability within each survey unit.
Tomorrow, we will conclude our discussion of experiments with an overview of ceramic fabric categories. Stay tuned!