A Method for Measuring Relative Abundance of Fragmented Archaeological Ceramics - John E. Byrd and Dalford D. Owens, Jr

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A Method for Measuring Relative Abundance of Fragmented Archaeological Ceramics

Author(s): John E. Byrd and Dalford D. Owens, Jr.Source: Journal of Field Archaeology, Vol. 24, No. 3 (Autumn, 1997), pp. 315-320Published by: Boston UniversityStable URL: http://www.jstor.org/stable/530687

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315

John E. ByrdEast Carolina University

Greenville, North Carolina

Dalford D. Owens Jr.University of Tennessee, Knoxville

Knoxville, Tennessee

Archaeologistsommonly se imple ounts f specimenss a measure f relative bun-danceforvariousfra,gmentedrchaeological aterials. imple ounts, owever,reprone o errordue to thedifferential ize offragments.A more alid measure hat ap-proncimatesurfacearea is theeffective rea (EA). Thismeasure ffers simple olutionto theproblem f quantifying eramicsfrom rchaeologicalontexts.

Introduction

Archaeologists have long been using simple sherd

counts as a measureof the relativeabundanceof ceramic

types (cf. Kidder 1931; Ford 1952). The simple sherd

count is derived by counting the fragmentsof broken

ceramic vessels. Researchersusing simple counts often

encountererrorsdue to varying herd size (Orton, Tyers,and Vince 1993; Childress 1992; Ford 1952; Gifford

1951, cited in Baumhoffand Heizer 1959), particularly

when sherds of one type are highly fragmentedwhile

others are not. In most cases it is assumedthat varying

sherdsizes will have a minimaleffect on the analysis e.g.,

Ford 1952; Childress 1992). Some archaeologists e.g.,

Orton, Tyers, and Vince 1993: 21; Chase 1985: 213;

McNutt 1973: 45; Solheim 1960: 325; Gifford 1951:

223, cited in Baumhoff and Heizer 1959: 309) have

expressedconcern with this assumptionand have ques-

tioned the use of sherdcounts as a measure f the quantity

of pottery (Orton, Tyers, and Vince 1993: 21; Chase1985: 213).

A central heme of this paper s that sherdcounts havea

low degree of validity (Nance 1987) as a measure of

relativeabundance.A measure'svalidity s determinedby

how well it measures what it is intended to measure(Nance 1987: 280); thus, it is necessary o state that the

assumptionhere is that measures of the relative abun-

dancesof specificclassesof ceramicmaterials re generally

intended to measure he quantityof ceramicmaterialbe-

longing to each class (not necessarilyhe numberof items

such as pots or bowls).

Orton, Tyers, and Vince (1993: 23) have identifie

three important ypes of evidencecommonlyprovidedby

ceramicanalyses n archaeology: atingevidence,distribu

tional evidence,and evidence or function and/or status

The full developmentof these areasof research nvolvethe use of measuresof relativeabundance. n sites where

ceramics replentiful, hey are often the primarymeansof

dating the context in which they are found as well as the

materials ecovered n association.Date rangesareassigned

based on the proportionsof types with known tempora

distributions.Distributions f ceramic ypescan shed light

on specificaspectsof past lifeways uch as tradepractice

(Orton, Tyers,andVince 1993: 23). An effectivemeansof

analyzing ype distributiondata s to present he data n a

quantitative istributionmapwhereproportions f respec

tive types at various ites are displayed Orton, Tyers,and

Vince 1993: 201). Information elating o the functionofindividualpots can naturally ead to an interpretation f

site function when proportions of different functiona

types present in an assemblageare ascertained Orton

Tyers,andVince 1993: 29).

Gifford (1951) was one of the earliestresearchers o

attempt to improve upon the simple sherd count as a

measureof relative bundance.He employed herdweigh

and found that "weighing sherds seems to give more

accuratestatistical esults than counting them" (Gifford

A M e th o d f o r Measuring R e l a X d v e

Abundance o f Fragmented

Archaeological C e r a m i c s



316 RelativeAbundance f Ceramics/Byrdnd Owen

1951: 223, cited in Baumhoff and Heizer 1959: 309).

Solheim (1960) also attemptedto utilize sherd weight,

pointingout thatrelyingon artifact ounts alonehas ed to

"loss of much informationotherwise nferable rom pot-

sherds,and this maybe true for other kindsof artifacts s

well" (Solheim1960: 325). The use of sherdweight as a

measureof relativeabundancedoes appear o be a signifi-

cant improvementover the simplecount (Orton, Tyers,

and Vince 1993: 166-181). The densityof ceramicmate-

rial, however,dependsupon designvariablesuch as tem-

per; heweight of a sampleof sherdsof a given type s only

partlydeterminedbythe quantityof pottery n the sample

(Orton, Tyers, and Vince 1993: 169). This can be a

problem when comparing,for example, fiber-tempered

sherds to sand-temperedsherds. One solution to this

problem s to determine he meandensities or sherdsof

specific types and defineconversionformulaethat make

the weightscomparable.The leachingof certaincompo-

nents ofthe paste (e.g., shell temper)will alsodetract rom

the degree of validityof weight as a measure,as will

varyingvesselwall thicknesses etweentypes.

Orton (1975; see also Orton,Tyers, andVince 1993)

has proposed hat the "vessel-equivalent"e adoptedas an

improvementover weights andsherd counts. The vessel-

equivalent s basedon the fact that every sherdis some

proportionof the whole pot fromwhich t originated.The

estimatedproportions assignedas the value or the sherd.

One wayto determine he vessel-equivalents to divide he

sherd weight for a typeby the known weightof a wholevessel (Orton, Tyers,and Vince 1993: 172). This proce-

dureworkswell when an acceptable alue or the weight of

a whole vesselcan be obtainedand the biasesof weight as

a measure reovercome.The meanvesselweightfor whole

vessels of a type could serve in this capacity,although

adequate amplesof whole vessels or the types of interest

are not alwaysavailable.An alternativemeansof obtaining

vessel-equivalentss to use only sherdswhoseproportions

of whole pots can beestimated e.g., rims,handles,bases)

as a representation f the whole pot (Orton, Tyers, and

Vince 1993: 172) The validityof this technique s poten-

tially essened by the tendencyfor the vesselpart selectedfor analysis o showdifferental survivorshipn the assem-

blage (Orton, Tyers,and Vince 1993: 174).

Employing implecounts as a measureof relativeabun-

dance gnores he factthat countsaregreatly nfluencedby

the degree of fragmentation.Weights are partly deter-

mined byvaryingdesign characteristicsf typesandvessel-equivalents retoo laborious o apply n manyarchaeologi-

cal research etangs.The following s a briefdescription f

a technique or measunng he amountof ceramicmaterial

that is simple to applyand free of the problemsencoun-

teredwith more traditionalmeasures.

Fragmentation nd Sherd Count

The mathematical elationshipbetween fragmentatio

and sherd count can be describedverbally.Considera

hypothetical ase n which a single vessel s broken n half

then the two fragmentseach break in half to produc

4 sherds, he 4 sherdsbreak nto8, the 8 sherdsbreak nto

16, the 16 fragment nto 32. We are observingan expo-

nential increase n sherd count with each fragmentatio

event, along with a concomitantdecrease n sherd size

The relationshipbetween sherdcount and fragmentatio

in the above example where n each fragmentationven

the sherdsbreak n half) s

N= 2x

where N is the sherdcount and X is the fragmentatio

episode.The number2 results rom the sherdsbreakin

into 2 pieces with each fragmentationpisode. Thus, we

begin with no fragmentationand a sherd count of 1,

which is predictedby 2°. The thirdfragmentationpisode

should produce 23 = 8 sherdsand the fifth episode wil

produce 25 = 32. These numbersmatch those given for

the hypotheticalexample.Note that as the sherd count

increases, he sherd size decreasesexponentially nd the

amount of ceramicmaterial emains he same.

In reality sherdsdo not necessarilybreakinto a set

numberof fragments such as 2) with each fragmentatioevent. Nevertheless, t is this exponentialncrease n the

numberof fragmentsas they fracture nto smaller and

smallerpieces that distorts the simple sherd count and

compromises ts validitywhen applied o assemblageswith

variableherdsize.

Methods

Surface rea s one attribute f ceramics hatcan provid

a reliablemeasureof relativeabundance McNutt 1973:

45; Chase1985: 218; Childress1992: 39). Advantages o

using surfacearea nclude he fact that it is not affectedby

external ffectssuch as fragmentation, hichdistortsrelative abundances etermined om sherdcounts,or varyin

paste characteristicsf ceramictypes, which can affec

relativeabundances igured fromsherd weights. On the

other hand,measuringhe surface reaof individualherd

and whole vesselscanbe an arduous ask(Childress1992:

39; Chase1985: 218) and will not likely be adopted by

archaeologistsnlessa relativelyimple echniques devel

oped for obtainingsuch data. The techniquedescribe

below can be used to obtain an estimateof surfacearea





Table2. PercentageEA per seriesfrom TestUnit C, DavenportSite, 31BR39.

Croaker Mnrsey

Contenct EA Mt. Pleasant Moskley Deep Creek Landin,g Creek Unknown Total

Zone I 13.75 2 4 79 13 2 0 100

Zone II 25.25 4 8 62 16 10 0 100

Zone III

Level 1 31.75 3 25 60 12 0 0 100

Level 2 6.0 0 16 48 36 0 0 100

Level 3 111.75 0 33 23 42 2 0 100

Level 4 41.25 0 15 7 78 0 0 100

followthe Robinsonpatternof monotonicdecrease rom

the principlediagonal Marquardt978).

The similaritymatrices re given in Table3. It is readily

apparenthat the IA values n the matrixcalculatedwith

the EA (TABLE 3) decreaseneatly off the diagonal n all

directionswhilethose in the sherdcountsimilaritymatrix

(TABLE3) do not. The EA dataproduce he correctorderof

the excavatedunits, while the sherdcount matrix ncor-

rectlyordersZone II beforeZone I.

The RelaiionshipBetweenEA and SurfaceArea

Childress 1992) has discussed he problemof compar

ing type frequenciesepresented y sherdcountsto those

represented ywholeceramic essels.He convertedwhole

vesselquantitieso measuresomparableo sherd requen

cies by estimating he surfaceareasof whole vesselsandthen dividingby the mean sherdsize to get an estimated

numberof sherdspertype(Childress 992: 39). Childres

listsone potential ourceof errorarising romthe variabil

itl in sherdsizein the samplesromwhichthe meanswere

Table 3. Similaritymatrices esulting rom frequencyseriationof excavationunits.Matricesbasedon simple

countsor on effectiveareavalues.

Seriation A B C D E F

Simplc countsA 200 189 134 130 90 50

B 189 200 133 127 87 49C 134 133 200 182 140 96

D 130 127 182 200 154 110

E 90 87 140 154 200 144

F 50 49 96 110 144 200

Effective area

A 200 166 157 130 84 48

B 166 200 165 144 98 62

C 157 165 200 153 121 69

D 130 144 153 200 150 116

E 84 98 121 150 200 128

F 48 62 69 116 128 200

318 RelativeAbundgneef Ceramics/Byrdnd Owen

(EgloffandPotter 1982;Phelps1983; Egloffet al. 1988).

The sherds or eachtype were sortedthrougha screenset

containing4, 3.5, 3, 2.5, 2, 1.5, 1, and0.5 in screens.The

EAvalueswerecalculated nd usedto obtainthe percent-

age representationf eachtype perlevel TABLE 2).

A chi-squareest for goodnessof fit was applied o the

ceramicdatawhere the percentagescalculated rom EA

values were used to derive expected values for sherd

counts. The test results(%2= 204.86, p = 0.00) clearly

indicate that the relativeabundancesbased on EA are

significantly ifferentfrom those determined rom sherd

counts.Sherdcount percentages nd EApercentagesrom

the excavatedunits in Test Unit C were next used to

conductfrequencyeriationso order he respectiveevels.

Frequency eriation echniqueswere developed or order-

ing archaeological nits for whichthere existedno othermeans of relative dating, such as stratigraphic ontext

(Dunnell1970;Marquardt978). The ceramicypes rom

the levels n TesttJnitC arealready emporally rderedby

their stratigraphic ositions as well as by researchcon-

ductedat other sites. It is assumed problemswith seria-

tion norwithstanding, f. Dunnell 1970) that if one or

both of the measuress not providinga validmeasureof

relativeabundance, frequency eriationusingthat meas-

ure will not orderthe levelscorrectly.This independent

check s intended o permitone to determinewhether he

relative bundancesbtainedusingsherdcountsor EA are

moredesirable.Frequencyseriationswere conductedwith the sherd

count data and EA data usingGelfand'sMethodII (Gel-

fand 1971; Marquardt1978). The method involvesthe

constructionof similaritymatrices,basedon Robinson's

(1951) index of agreementIA)for eachpairof units.The

indexof agreementbenveen eachpairof unitswascalcu-

latedby subtractingrom200 the summedabsolutevalues

of the differencesbetweenthe percentagesorthe respec-

tive types, where 200 represents he maximumpossible

agreement.The order foreach row shouldbe ordered o



Journollof FieldArcholeology/Vol.4, 1997 319

calculated,as indicatedby the large standarddeviations(Childress1992: 39).

EAvaluesof sherds,alongwithestimates f wholevesselsurfacearea,makepossiblea simpler,more accurateap-proachto the problemof convertingsherds o a measure

that can be combinedwith whole vesselquantities.Thisapproach estson the factthat EAcan be predicted romsurfacearea.Ceramicvessels of ten differentshapesandsizes were assembledto investigate he relationshipbe-tween EA andsurfacearea.Surfacearea orallvesselswasestimated(in squareinches) and the vesselswere sub-sequentlybroken o varyingdegreesand the EAmeasured.Surfacearea and EA for the threevessels n the previousexperimentwereincluded.

A simple inearregression nalysiswasdonewithsurfacearea as the independentvariableand EA as the responsevariable.Because thereare severalvaluesof EA for eachvalue of surface area,it is necessary o formally est forlinearity see Zar 1984: 282). The one-tailedF-test forlinearitydescribed n Zar(1984: 282) wasused to test thenull hypothesis hat the dataarelinearlydistributed.Thenull hypothesiswas accepted(F = -3.92, p > 0.25). Theregressionanalysis esulted n a correlation oefficientof0.996 (R2= 0.991) andanoverall ignificanceevelof p =0.000 (F = 4666.279). It is clearthat total EA for anarchaeological ssemblage an be effectively stimatedbyemployinga regressionmodel to convertvessel surfaceareas o EA.The modelpresentedhere FIG. 1),

EA = 0.831 (surf6lceIre6l)is generallyapplicable o most ceramicvessels, but moreprecisemodels can be calculatedby using dataobtainedfromvessels of the specificshapesof interest.

Discussion

Analyticalproceduresn archaeologyhave traditionallyinvolvedusingsimplesherdcountsforrespectiveypesasameasureof relativeabundance.Whensuch a proceduresappliedto the experimental ypes above, it resultsin adistorted picture of the ceramicassemblagewhere therelativeabundancesarered 74.5%,black

22%,andwhite3.5%.These percentages uggest that thereareconsider-able disparitiesn the quantitiesof ceramic n each type.Percentagesbased on EA producea drastically ifferentpicturewith the red composing30%of the assemblage,while blackandwhite constitute35% ach.It is clear hatEA provides a more accuratemeasureof relativeabun-dance.

The archaeologicalpplicationncludeda statisticalestthat showed that relativeabundancesbasedon EA weresignificantly ifferent romthose basedon simplecounts.

250

200

150

:LL

100

50

o0 100 200 300

SA

Figure 1. Regressionmodelwith EApredictedrom surfacearea hatwascalculated iom anexperimentalssemblagewithvesselsof vary-ing sizesandshapes.

This findingverifiesthe impressionof the authorsthatsherdsarenot consistently he samesize but vary,some-iimes considerably.The frequencyseriaiionusing sherdcounts showsthatvaryingsherdsizescan indeedlead to

errorsin interpretaiion hat are archaeologicallyignificant.

Acknowledgments

The authorswould like to thankthe editor,RicardoJ.Elia,andthe anonymous eviewers ortheirmanyvaluablcomments on this paper.R. P. Stephen DaxTis, r. andRobertC. Dunnellarealsoacknowledged orcommentingon earlier ersionsof thiswork.The contributions f all oftheabovehavegreatlymprovedhispaperbutresponsibiityfor anyshortcomings estssolely uponthe authors.

John E. Byrd(Ph.D. University f Tennessee,Cnonsville,1994) is a Reseolrchssociolten theInstitute or Histori-cal olndCulturollReseolrchlndVisitingAssistolnt rofes-sor in theDepolrtmentfAnthropologyltEolstColrolinoUniversity.Molilingolddress:epolrtmentfAnthropol-osgy, olstColrolinol niversity,Greenville,NC 27858.

DollfordD. Owens r.is a grolduoltetudent n olnthro-pology lt the University f Tennessee,Cnonsville.olilingolddress:epolrtmentfAnthropology, niversity f Ten-nessee,Cnoxville,Cnonsville,N 37990-0220.



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Documents

A Method for Measuring Relative Abundance of Fragmented Archaeological Ceramics - John E. Byrd and Dalford D. Owens, Jr