Subjectivity Inherent In By-Eye Symmetry Judgements and the Large Cutting Tools at the Cave of Hearths, Limpopo Province, South Africa

Centre for the Archaeology of Human Origins, University of Southampton

The Stone Age of South Africa is an area of study due for a renaissance, and there is a real need for unification of the extant evidence. As a beginning to this, new methodologies have been proposed. This paper tackles the issue of symmetry, specifically the subjectivity involved in by-eye judgements. Assumptions of subjectivity, however, are not proof: presented here is a critical analysis of the inherent bias of by-eye symmetry judgements. Ultimately it is clear that the method contains a level of subjectivity which strips it of any analytical value. The by-eye judgement of symmetry is replaced by the more robust Flip Test computer program, and a brief study is made of the Large Cutting Tools (LCT) at a vitally important, yet often overlooked, site dating from the Pleistocene in South Africa, the Cave of Hearths, Limpopo province. The corollary is that the symmetry present in the Cave of Hearths Large Cutting Tools can be studied with some measure of confidence: suggestions are made regarding the nature of tool typologies and the knappers’ ultimate focus on tip shape and utility.

**Keywords**

ANOVA, by-eye, Cave of Hearths, Flip Test, Large Cutting Tools, subjectivity, symmetry

Introduction

The starting point of this paper is McNabb, Binyon and Hazelwood’s (2004) “The Large Cutting Tools from the South African Acheulean and the Question of Social Traditions”. Despite my disagreement with several aspects of their theoretical stance (Underhill-Stocks 2006; see also Palmer et al. 2005 for a similar view) it is their ‘by-eye’ symmetry methodology which is the focus here. Although newly designed their methodology received little attention in the subsequent comments. Only Machin and Mithen (2004) really focus on the methodology, criticising the symmetry measure from which McNabb et al. ultimately make their suppositions about symbolism. What Machin and Mithen objected to most strongly is the assumed subjectivity involved. However, their dissatisfaction is based on this assumption, with no investigation into the proposed subjective ambiguity within the method. Presented here is a statistical analysis of the by-eye symmetry methodology, focusing on potential levels of subjectivity within it. Ultimately it is clear the method does indeed contain a level of subjectivity which strips it of any analytical value. Following a brief critique of available alternatives the results of an examination utilising Hardaker and Dunn’s (2005) Flip Test is presented to facilitate discussion of the levels of symmetry present within the Cave of Hearths Large Cutting Tools.

A focus on symmetry is important in the present climate of burgeoning cognitive archaeology, due to its accepted relationship with cognitive development (McBeath et al. 1997; Tversky and Lee 1998); as such it may allow communication between the archaeological record and cognitive sciences, a partnership which should elucidate the development of hominin cognition (Wynn 2002). Through studying changes in both standardisation and symmetry it is believed that one can trace the origin and evolution of the ‘human’ mind (Nowell et al. 2003). One important aspect not considered here is the influence of any re-sharpening and reuse on an artefact’s eventual symmetry. However, McNabb et al. (2004: 668) assert that re-sharpening was “not particularly important to the Cave of Hearths knappers” with only the bare minimum of secondary working present. The local abundance of raw material makes it unlikely that any of the artefacts were curated, and so any re-sharpening will have little effect on the overall patterns revealed.

The By-Eye Symmetry Methodology

The bilateral symmetry measure established by McNabb et al. (2004) (see also Sinclair and McNabb 2005) involves dividing each tool into equal thirds bisected along the long axis, which is established through the presence of a clear convergence, or by passing through half of the maximum width. This creates six sections, three either side of the bisecting line: the tip, medial and basal portions. To establish the extent of any symmetry these sections are imagined folded around the long axis and scored via a binary yes/no system. As each tool consists of three yes/no scores it can be assigned a category based on the eight possible combinations. In addition McNabb et al. (2004: 658) felt it necessary to examine three further relevant classes; near-symmetry was scored for those tools which where not manifestly symmetrical but still retained a “sense of balance”, defined as the knapper evidently paying attention to “the distribution of volume around the midline when seen in planform”. Visually distinctive features, which occur in parallel on opposite edges, were noted as relevant to overall symmetry; for example, notches which initiate the edges sweep toward the tip. Finally, they marked profoundly asymmetrical tips, which evidently resulted from working, as it was the knapper’s decision whether to impose clear bilateral symmetry or not.

Intuitively this appears subjective, which is what Machin and Mithen (2004) object to. Unfortunately, they appear to misunderstand the overall aim of the methodology. Whilst remaining acutely aware of the need to extract patterns from infinite variability McNabb et al. attempted the construction of a *simple* methodology to enable the re-analysis of the huge number of artefacts present in the South African Acheulean, in order to facilitate the unification of the evidence and allow more effective study to progress. As they themselves state, “we aimed at simple comparisons of assemblages of large cutting tools on an intrasite as well as intersite basis” (McNabb et al. 2004: 656). However, it was patently vital to maintain a level of simplicity to enable the methodology’s uptake. Unfortunately, whilst by-eye judgements allow for a swift turnover in the number of artefacts which can be studied, it does seem unlikely that two unconnected researchers could replicate the same results. Its reliance on pure observation potentially exposes a minefield of biases (Adams and Adams 1991). However there is no proof that this particular method actually does suffer from this. What follows is an investigation partly extracted from the author’s MA dissertation (Underhill-Stocks 2006).

Subjectivity of the By-Eye Measure

To investigate the possibility of observer bias eight independent participants analysed digital images of 50 bifaces taken at random from the Pinel 6 sample in the Marshall et al. (2002) database. A larger sample base would have been desirable but the ad hoc distribution of the test was precluded by the wish to retain a variable demographic: five were graduates (four in archaeology and one in health sciences), three of which were trained in Lithic Analysis, with ages ranging from 24 to 58. The biface images were prefaced by the McNabb et al. symmetry methodology, extracted verbatim from their paper with no further instruction given to any of the participants. Each example was accompanied by a basic table for recording the binary yes/no scoring, in addition to near-symmetry, features promoting symmetry, and tip asymmetry. As an interesting aside even those not trained in lithic analysis, or archaeology, were startled by just how subjective the test felt, yet a belief in subjectivity is no proof of it, which is the entire point of this particular examination.

The results were statistically analysed utilising an Analysis of Variance (Miller 1986; Thayer and Turner 2001). Although SPSS (Version 14.0 used here) has the ability to run this test, it is necessary to understand the process in order to make use of its results. This method examines the results through two groupings: each participant’s 50 results, and each individual artefact’s 8 results generated following the McNabb et al. method. The null hypothesis, that the test is not affected by observer bias, predicts that regardless of participant the artefact’s score should be the same, producing a variance ratio of 1:1 (F distribution). To calculate this, one must first know how the question itself limits the possible variance (degrees of freedom or *df*). As there are two groups each will affect this in its own way (see Sheskin 2004). Next one must know how each result deviates from the group mean. The calculation (∑(Xi - X)^{2}) ÷ *df* for each group will give the population variance. These groups are then divided to gain the F distribution. If the null hypothesis is correct this should equal one, or a confidence limit of .95, representing that the result is statistically significant (alpha or α value).

Results of Subjectivity Study

The following table represents the full Analysis of Variance for the test on McNabb et al.’s symmetry methodology. This result displays an alpha value of 0.001 clearly revealing that the variation cannot be chance alone: each participant’s subjectivity must have affected the artefact scores, confirming that the bias of the test undermines its ability to reveal true patterns.

It was deemed important to retain a variable demographic to investigate the mitigation of subjectivity. Although not an ordinal scale the mean of each participant’s symmetry total for the assemblage shall be used as a proxy for differences in their perceptions. Foremost in importance is the presence or otherwise of lithic analysis training: a mean symmetry for the assemblage of 3.75 ± 2.23 was established for those with training, compared with 3.95 ± 2.24 for those without. Therefore, as confimed by an Analysis of Variance there was no statistically significant difference between the two groups. Further examination reveals there was in fact no pattern. The largest disparity exhibited is between participants with a general academic background and those without; a mean of 4.08 and 3.53 was found respectively (α = .016). All of this points to the inescapable conclusion that there is no way of mitigating the subjectivity of by-eye symmetry judgements.

Sum of Squares |
df |
Mean Square |
F |
Sig. |
||

TIP |
Between Groups | 11.23 | 7 | 1.604 | 7.104 | 0 |

Within Groups | 88.52 | 392 | 0.226 | |||

Total | 99.75 | 399 | ||||

MEDIAL |
Between Groups | 24.898 | 7 | 3.557 | 19.67 | 0 |

Within Groups | 70.9 | 392 | 0.181 | |||

Total | 95.798 | 399 | ||||

BASAL |
Between Groups | 15.79 | 7 | 2.256 | 10.65 | 0 |

Within Groups | 83 | 392 | 0.212 | |||

Total | 98.79 | 399 | ||||

TOTAL |
Between Groups | 303.47 | 7 | 43.353 | 10.02 | 0 |

Within Groups | 1696.28 | 392 | 4.327 | |||

Total | 1999.75 | 399 |