Temporal uncertainty and artefact chronologies

Computer Applications and Quantitative Methods in Archaeology: Session: Embracing uncertainty in archaeology

Southampton, 28 March 2012

Andrew Bevan (with contributions from many colleagues)UCL Institute of Archaeology

Chronology and Periodisation

• Lots of work emerging in other disciplines about space-time analysis and how to handle uncertainties associated with the timing of events

• Archaeological time poses its own peculiar challenges… a native archaeological tradition associated with radiocarbon dating (and mapping) but the handling of categorical dates now increasingly considered as well

Aoristic Analysis

Ratcliffe 2000 Int. J. GISJohnston 2003. CAACrema, Bevan, Lake 2010 J. Archaeological ScienceCrema in press J. Archaeological Method and Theory

• A probability of presence of an event, site, artefact is assigned to a series of time-block, usually based on defining start and end dates, with a uniform distribution in between.

• Works well for existing archaeological datasets and supports Monte Carlo simulation

• Typically, done long after data collection, when some information is already lost …

• Aoristic time-blocks are often equal and absolute …

• …mixes up the uncertainty of attribution to a time period (a relative scale) with the fixing of this period in absolute units (e.g. to a range in years BCE)

Intensive Surface Survey on Antikythera

• Directed by Andrew Bevan (UCL) and James Conolly (Trent University, Canada), in collaboration with Aris Tsaravopoulos (Greek Archaeological Service)

• Stage-one survey by walkers spaced 15m apart• Stage-two survey of certain localities on a 10x10 grid.

Uncertainty in Assigning an Artefact to a Period

Initial impression of date:“possibly Hellenistic…more likely Late Roman…but not in between”

Perhaps recorded in a traditional database as: “Late Roman”, “Late Roman?” or“Late Roman/?Hellenistic”

Jar handle

An alternative is “percentage confidence”… 30% Hellenistic, 70% Late Roman

Bevan, Conolly, Hennig, Johnston, Quercia, Spencer, Vroom in press. Archaeometry

First or Second Palace

Late Roman

Diagnostic Confidence by Period

Sherds with ≥70% confidence of being Middle Byzantine to Early Venetian (c.1000-1400 AD).

Mapping Diagnostic Confidence

Sherds with ≥70% confidence of being Middle Byzantine (c.1000-1200 AD)

Sherds with ≥20% confidence of being Middle Byzantine (c.1000-1200 AD)

Close-up (diameter 600m) with percentage confidences of Middle Roman date shown as graduated colours overlain on all other sherds (in grey)

All possible candidate sherds for a Middle Roman date (i.e. >0% confidence),

Mapping Diagnostic Confidence

Overall Uncertainty

€

U j = min Pij ,maxk≠j Pik( )( )i =1

n

∑ Piji =1

n

∑

Example for period 2:(40+10+0+20+0) / (40+10+0+20+0) = 1

and for period 3:(40+10+0+20+0) / (50+10+0+20+100) = 0.389

Pairwise Uncertainty

€

U jk = min Pij ,Pik( )i =1

n

∑ × 2 Pij + Pik( )

Example for period 2 and 3:(40+10+0+20+0) x 2 / (90+20+0+40+100) = 140/250 = 0.56.

First or Second Palace

Late Roman

Relative chances of defining Middle Roman dates compared to Early and Late Roman

Local Uncertainty

Wider Implications for Archaeological Fieldwork

• Observer Variability – ability to explore intra- and inter-observer variability

• Permanent Collection – ability to demonstrate added value of the collection and long-term storage of artefact assemblages.

• Physical Re-investigation – can be used to design strategies for new fieldwork

• Regional Profiling – supports construction of a regional uncertainty profile and exploration of case-by-case departures from it.

Thank you….

…and to those involved in…The Antikythera Survey Projectwww.ucl.ac.uk/asp

…and on behalf of ASP to…The Greek Ministry of Culture The Canadian Institute in Greece

Datasets available as ADS Collection 1115http://dx.doi.org/10.5334/4f3bcb3f7f21d