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Friday, 1 June 2012

The Caveman Effect

  Pentre Ifan -- a place of special sacredness, or simply one of many, of which most of the 
others have been removed?  In other words, simply a cromlech that nobody had any 
particular reason to get rid of?  A random relic rather than a supreme example of megalithic engineering?


As readers of this blog will know, one of my favourite themes is sampling bias.  I have recently been trying to explain to Geo that there is not necessarily any great "significance" in the fact that a few old geologists found a few erratics from a certain area in another area where there happened to be a few ploughed fields which were easy to examine.  On that basis they described what they found, thereby inviting other geologists to go and have another look in the same place, some 70 years later -- that, in a grotesque sort of way, invested the location with even more "significance" simply because it was mentioned by name...........


But actually, our concentration on this particular site is entirely illogical and artificial........ it may well be that there is a far more significant site one field away, or a kilometre away, or ten kilometres away, which nobody has yet discovered or got round to looking at........


This is a quote from the Wikipedia page on sampling bias:
An example of selection basis is called the "caveman effect." Much of our understanding of prehistoric peoples comes from caves, such as cave paintings made nearly 40,000 years ago. If there had been contemporary paintings on trees, animal skins or hillsides, they would have been washed away long ago. Similarly, evidence of fire pits, middens, burial sites, etc. are most likely to remain intact to the modern era in caves. Prehistoric people are associated with caves because that is where the data still exists, not necessarily because most of them lived in caves for most of their lives.

 One of the Lascaux cave art images.  Were the painters cave dwellers or did they live in the open?  Were they better painters than those who painted things outside the caves?  Were the Lascaux artists uniquely clever?  Lots of questions, and lots of room for sampling bias......

I have previously admitted to sampling bias myself,  when in my D Phil thesis I gave great importance to a site called Ogof Golchfa, thinking it to be unique and hugely important, whereas when I went back many years later I realised that there was an even better (and more important) site just round the corner, which I had entirely missed.  Why had I missed it?  It might have been covered with brambles at the time, or it may have been pouring with rain when I walked past it, or it may only recently have been revealed by a rockfall.

On the same basis I have expressed concerns about the incredibly detailed provenancing by Rob Ixer and Richard Bevins of the "foliated rhyolites" from Craig Rhosyfelin to within a few metres of one particular sampling location.  As I have explained before on this blog, I am not sure that you can do that, unless you have sampled several hundred sampling points on a detailed and very tight grid, across a large area.  And your sampling is in some way biased towards the places where you have rock outcrops at the surface.   In other words, the provenancing conclusion may be down to sampling bias rather than reality.  So although their geology is highly impressive, I retain a degree of doubt and reserve judgment on the conclusions reached. 

In the archaeological field, we see over and over again that a site or group of sites may be interpreted as belonging to some great cultural complex, only for the thesis to be overturned as new discoveries show wider distributions than anticipated, or even larger and more complex associations of features elsewhere.  In Pembrokeshire, for example, we have suspected for many years that the distribution of cromlechs or standing stones (as we see them today) may have no geographical or cultural significance -- it may simply be that those features in the roughest areas have survived better, while features in the lowlands which have been heavily used for agriculture over the past few centuries have been flattened or simply removed because of their "nuisance value."  And those features that have survived in the uplands have not survived because they were venerated in any way, or even because they were erected in "sacred places" -- but because in recent centuries they were not particularly in anybody's way, so that nobody could be bothered to remove them.

7 comments:

Geo Cur said...

And I have tried to explain to you that the interesting point is not only the chemistry of the erratics but the situation of the sampling ,near the coast , having avoided the overland trudge , and also close to where you suggested was a more obvious route for embarcation .

Tony Hinchliffe said...

It is the case in Wiltshire that, with the advance in techniques for discovering relics of barrows which had been substantially ploughed out, we now know there were more in the river valleys than previously thought, e.g. along the [Bristol] Avon valley.

Anonymous said...

I've been spotting stones that seem out of context and/or that are standing on my walks all over North Pembrokeshire (and are not marked on my OS map). I can't believe that they are are all glorified cow bottom scratchers or ancient gate posts.

Strangely I tend to see more since having bookmarked your blog.

Davey

BRIAN JOHN said...

Good to hear of a man who has an educated eye...

Anonymous said...

check with Olof's book he too found unrecorded stones.
M

Geo Cur said...

Note the “caveman effect “on the sampling bias wiki needs a citation , probably because it is not a very good analogy and hardly appears to have been used very often with the same meaning . A sampling bias involves the exclusion of data that has been unwittingly or knowingly ignored , in the case of an inventory cave art sites nothing has been ignored but there are obviously examples that we are unaware of , and others lost . To apply the term to cave art or an inventory of megalithic monuments is similar to accusing a pollster of sampling bias because they didn’t include dead people in their poll .

Tony Hinchliffe said...

A similar example of what I might call "naked eye sampling bias" may have occurred in relation to Stonehenge and the provenance of the sarsens within the County of Wiltshire.

Sure, there aere loads of sarsen stones to be found to the east of Avebury henge and circles, particularly on Overton Down, but, the "naked eye samplers" have leapt to the false conclusion that, because there are, or have been, loads of sarsens or 'grey wethers' between Avebury and Marlborough, the Stonehenge sarsens MUST have been sourced from that part of Northern Wiltshire. That area also became famous beyond Wiltshire 150 - 200 years or more ago as the supply zone for much building stone for London and S.E. England, particularly after the development of railways.

But there is historical evidence for a quite plentiful supply of sarsen stones in the general Stonehenge/ Amesbury vicinity, and this has at last been acknowledged in the last few years by no less a body than English Heritage. Moreover, as has been said in previous comments, there still are sarsen stones in the ground, e.g. north of Stonehenge, near the early Neolithic causewayed camp called Robin Hood,s Ball; and the various single sarsen orthostats such as the Cuckoo Stone and the Bulford Stone. Furthermore, the Heel Stone is also reckoned to have been positioned close to its natural original location in the landscape.

Yet it is so easy for beliefs, which may have been largely attributable to perhaps just one person, to then be repeated ad infinitum and, before you know it, they have become widely accepted and thus so-called "common knowledge". However, it is as well to remember that not all folklore is verifiable as fact.