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1987 13: 65Journal of Information ScienceE. Ray Stinson and F.W. Lancaster

citation studiesSynchronous versus diachronous methods in the measurement of obsolescence by

  

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65

Synchronous versus diachronous methodsin the measurement of obsolescenceby citation studies *

E. Ray StinsonOffice of Sponsored Programs, University of Texas MedicalBranch, Galveston, TX, U.S.A.

F.W. LancasterGraduate School of Library and Information Science, Universityof Illinois, Urbana, IL, U.S.A.

Received 24 September 1986Revised 13 November 1986

Using the literature of human and medical genetics, theresults of a synchronous citation study of obsolescence over a19-year period were compared with the results of a di-

achronous citation study. If the first two years of synchronousdata are excluded, the rate of obsolescence measured synchro-nously is statistically equivalent to the rate of obsolescence

measured diachronously.The assumption that synchronous studies need to be cor-

rected to account for the growth of the literature was tested.The data collected support the hypothesis put forward byBrookes that the growth of the literature and the growth of thenumber of contributors to that literature have opposite effectsin the measurement of obsolescence. The results of a synchro-nous study corrected for the growth of the literature and alsofor the growth in number of contributors were statisticallyequivalent to synchronous data with no corrections whatever.

Strictly speaking, ’obsolescence’ refers to the

phenomenon of replacement. Thus, one can con-sider a domestic appliance to be obsolete when it

is superseded by a later (more powerful or moreefficient) model. Evidence of the obsolescence of

publications is presumed if the use of these pub-lications declines with age. Decline in the use of

literature over time (‘aging’ or ’decay’) may beascertained through studies of library use or bycitation analysis. Citation studies can be either

* The work reported here is based largely on doctoral researchcompleted by Stnson at the Graduate School of Library andInformation Science, University of Illinois.

synchronous or diachronous (Line and Sandison[6]). In the former, the publication dates of sourcescited in current publications are plotted back-wards in time to determine, for example, how farback one needs to go to account for, say, 50% ofthe sources cited (median citation age). In the

latter, a sample of items published in the past ischecked against citation indexes to determine therate at which citation declines with the age of the

items.

More precisely, Line and Sandison refer to

diachronous studies as those that follow the use of

particular items through successive observations atdifferent points in time, whereas synchronousstudies are concerned with plotting the age distri-bution of material used at one point in time.

Rate of obsolescence can be plotted as a curve.Alternatively, some numerical measure can be

used. For diachronous studies this may be the

’half life’: that period of time needed to accountfor one half of all the citations received by a groupof publications. ’ For synchronous studies the

comparable figure is the median citation age (Lineand Sandison [6]).

Synchronous studies are performed more oftenthan diachronous studies because they are easierto conduct. In fact, until the emergence of citationindexes in the 1960’s large-scale diachronous stud-ies were not really feasible. However, Line and

Sandison have claimed that there is no reason to

suppose that obsolescence measured synchro-nously will be the same as ’true’ obsolescence

measured diachronously. For example, there is noreason to suppose that the half life for some

subject is the same as the median citation age in

that subject.The need for a comparison of diachronous and

1 In point of fact, of course, a true half life cannot beestablished because further citations are likely to occur afterthe data have been collected. ’Half life’, then, really means"the half life observed at a particular point in time".

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synchronous obsolescence has long been recog-nized. Some studies have been undertaken (e.g.,Line et al. [7], Oromaner [10], Line and Carter [5],Griffith et al. [3], and Ferraz Barboza [2]) but theyare all very limited in scope (e.g., based on a smallgroup of journals only or involving very small

samples.)When citation studies are used to measure ob-

solescence. the growth of the literature and the

growth in the number of contributors can com-

plicate the issue. On the surface, the justificationfor correcting for literature growth in synchronousstudies seems obvious. The 1983 literature on some

subject may cite 1980 sources at twice the rate it

cites 1970 sources. However, if twice as much onthe subject was published in 1980 than was pub-lished in 1970, the 1983 citations merely reflect

probability and show no evidence of decline in usewith age.

Brookes [1], however, has disputed the need tocorrect for literature growth. He developed an

equation to show that the growth in number of

contributors to the literature on some subject willcancel the effect of the rate of growth of the

literature if both rates of growth are equal. Thejustification for this approach is that, as the litera-ture increases, the chance that any particular paperwill be cited decreases (because papers are com-peting with each other for available citation;>, but

the more authors that contribute to the subject thegreater the chance that a particular paper will bediscovered and chosen for citation by someone.Oliver [9] tested the Brookes hypothesis (whichhas not been widely accepted by other investiga-tors of the obsolescence phenomenon) by studyinga small sample of references in the literature of

semiconductor physics. It was found that, over a

five-year period, the number of articles increasedby 13.4% and the number of contributors in-

creased by 15%. Oliver concluded that no signifi-cant differences existed between the two variables

and produced preliminary evidence to suggest thatthe Brookes hypothesis is essentially correct.

The objectives of the research reported here

were

(a) to further test the Brookes hypothesis thatgrowth in the literature and growth in the numberof contributors will tend to balance each other outin obsolescence studies, and

(b) to compare synchronous and diachronousmeasures of obsolescence.

Growth considerations

The study was performed in the area of humanand medical genetics because this is a subject thatwas well covered in the early volumes of theScience Citation Index (SCI), as well as in the SCIprototype, the Genetics Citation Index. The field ofhuman and medical genetics was defined as articlesindexed under terms that relate to genetics in

Index Medlcus.

It was hypothesized that the literature would

grow at a fairly regular rate over a five-year periodof time. So, the growth was estimated on the basisof the number of articles that were indexed under

‘genetics’ headings in Index Medicus in 1960,1965, 1970, 1975, and 1979. The size of the litera-ture for the intervening years was extrapolatedfrom the data for these sample years.

Based on the descriptors appeanng in the 1980Aledical Suhject Headmgs (MeSH) in the G5-

Genetics, subcategory, together with those termslinked to these descriptors by means of ap-

propriate cross-references, a MEDLINE search

back to 1966 was conducted to retrieve items

having the publication dates 1965, 1970, 1975, and1979. All items in the database for these yearswere printed out if they had been indexed underat least one of the genetics terms used as a majordescriptor (i.e., the item would appear under a

genetics term in the printed Index Medlclls).Because the MEDLINE database that could be

searched routinely went back only to 1966, supple-mentary searches in Index Medicus were neededto complete the genetics literature count for 1965and to obtain a count for 1960. The 1965 Inde.i

A4edicus was searched for items having a publica-tion date of 1965 and indexed under a geneticsterm. In this case, the genetics terms were drawnfrom the appropriate categorized list in the 1966

MeSH, together with terms linked to these byappropriate cross-references. The 1965 items foundby the manual search were added to the 1965

items retrieved from the MEDLINE search. The

same procedure was used to determine the size ofthe literature for 1960. All terms identified in the

1966 categorized list were examined in the al-

phabetical section of the first edition (1960) ofMeSH. All valid terms and all cross-references tothem were examined in the 1960 and 1961 Indea

Medicus (for articles published in 1960). Uniquearticles that were listed in either the 1960 or 1961

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volumes were included in the bibliography of thearticles published in 1960 that dealt with humanand medical genetics.No claim is made that these procedures reflect

the true size of the human and medical geneticsliterature for the period studied because the

database used, while perhaps the one most com-prehensive, will not contain everything. It is the

rate of growth, not absolute size, that is importanthere. It was assumed that the growth as reflectedin the database of the National Library of Medi-cine would be a reasonable approximation of thetrue growth of the human and medical geneticsliterature.

From the bibliographies developed by these

procedures, it was also possible to make estimatesfor the growth in the number of contributors tothis literature, again based on the years 1960,

1965, 1970, 1975, and 1979.A random sample of 500 articles was drawn

from each of the bibliographies for years 1965,

1970, 1975, and 1979. For 1960, where there were

only 837 articles, a sample of 250 articles was

drawn. For each article in these samples, the fol-lowing variables were determined:

K, = number of papers in the population with

author i listed as the first author, .

E = expected number of first authors in the sam-ple,

L = number of unique first authors in the sample,nl = number of authors other than first authors

(subsequent authors) who appeared once inthe sample,

n, = number of subsequent authors who appeared-

twice or more in the sample,YJ = number of papers in year j,PI = number of papers in the sample from year j,~ = number of contributors in year j.

For n, and n,, subsequent authors who werefirst authors of other articles were excluded. In

addition, authors who appeared as subsequentauthors because the same article appeared morethan once were also excluded. The equations usedto determine the number of contributors for any

year were as follows:

Obviously, n 1 would be an underestimate and( n 1 + n 2)t would be an overestimate, but (~ 1+ n 2)t is a reasonable estimate of the number ofunique contributors to the field in any particularyear.

To determine a confidence interval. the largesample was divided into five equal random sam-ples ; the same variables were then determined andthe same equation was used for calculations. Theconfidence interval was calculated as follows:

with s as the standard deviation for the five

subsamples.After the number of articles indexed under

human and medical genetics terms in Index

Medicus for the years 1960, 1965, 1970, 1975, and1979 were tabulated, the growth of the literaturewas estimated. An average annual growth rate wascalculated for each five-year period and the size ofthe literature for the intervening years was calcu-lated according to the following equation:

.x,= size of the literature in year i,

_v, = size of the literature in year I + 1,a = average annual growth rate.

The literature grew from 837 articles in 1960 to1 591 articles in 1979. When the growth of the

YearFig. 1. Literature growth on linear scitle.

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Tahle 1

Growth in contributors

~ The number of contributors was measured in these years,

and the results were used to calculate the number of contribu-

tors for the intervening years.

literature was graphed (Fig. 1 ), it became apparentthat the growth rate was not exponential but closeto linear. The number of articles on genetics

Year

Fig. 2. Growth m number of contnbutors.

increased dramatically in the 1960’s, but began tolevel off in the 1970’s.

The estimated number of contributors to thehuman and medical genetics field from 1960 to

1979 is shown in Table 1. Figure 2 indicates thatthe growth in the number of contributors for thoseyears was nearly linear.

To compare the growth of the literature to thegrowth in contributors, both growth-rate curveswere placed on a scale of 0-100, in which 0

represented the lowest number of articles (contrib-utors) and 100 represented the highest number ofarticles (contributors). The following equation wasused: -

x, = adjusted number of articles (contributors) inyear i,

y, = number of articles (contributors) in year i,

z = minimum number of articles (contributors) in

any year,w = maximum number of articles (contributors) in

any year. ,

’

..

The adjusted growth of the literature and con-tributors is shown in Fig. 3. To evaluate whether

the curves were significantly different, the Kolmo-gorov-Smirnov test was used. The null hypothesisthat the two frequency distributions were equalcould not be rejected at the new a = 0.05 level.

Fig. 3. Growth of literature, and numhcr of contributors on a

numerical scale.

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Thus, there exists a 95% chance that the curves are

statistically equivalent.

Diachronous versus synchronous obsolescence

To obtain the rate of aging in the diachronousstudy, all citations to a sample of articles pub-lished in 1960 were traced; in the synchronousstudy, bibliographic references in a sample ofarticles published in 1979 were used. Each samplewas drawn from articles indexed under geneticsterms in Index Medicus for the appropriate years.

In the diachronous study, a random sample of500 articles was drawn from the 837 unique articlesthat were identified as contributing to the humanand medical genetics literature in 1960. Citationsto each of the 500 articles were tabulated, by year,through an examination of each volume of the

SCI (1961 to March-April 1980). A MEDLINEsearch identified 1118 genetics articles for 1979and a random sample of 200 of these was selectedfor the synchronous study. All but one of thesewas located and all their bibliographic referencesto journal articles were recorded and tabulated bydate of publication. References to other sources,including personal communications and unpub-lished works, were excluded so that the data were

compatible with the data from the diachronous

study. References to articles ‘in press’ were

Table 2

Number of source journal items used in SCI /

included for the year that was specified as the

anticipated time of publication; when no year wasspecified, 1979 was recorded.

The 500 articles in the diachronous studyreceived 13 734 citations between 1961 and 1979.

The data were analyzed three ways: with no cor-rection factor; with a correction for the growth ofthe SCI; and with a correction factor for the

growth of the literature.Increases in the coverage of a citation index

may affect the results of a diachronous study,distorting data collected on decline in citation

with age. The SCI has expanded greatly since itsfirst publication in 1961 as shown in Table 2. Thisgrowth reflects both the growth of the literatureand the increased coverage of the SCI. Because

these two variables should be separated in

obsolescence studies, a correction was made for

the increase in coverage of the SCI by usingTable 2 and the following equation:

.i = normalized number of citations to the popula-tion of articles,

y = citations to the population of articles in the

relevant year,- = source journal items in the relevant year,113 318 = source journal items in 1961.

x, = corrected number of citations in year i,

Y, = number of citations in year i,

=, = size of the literature in year i,

837 = size of the literature in 1960.

To compare the rates of obsolescence indicated

by each of the diachronous measures, correctedand uncorrected, the number of citations used was

adjusted to a scale of 100, with 0 corresponding tothe minimum number of citations and 100 to the

maximum number of citations. The followingequation was used:

- , = adjusted number of citations in yearsy, = number of citations in year i,= = minimum number of citations in any year,w = maximum number of citations in anv year.

The rate of decline in citation for each of the

methods is shown in Fig. 4. The rate of decay was

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70

Fig 4 Results from diachronous study.

great at first but began to level off after about sixor seven years. The half life was six years when no

correction factor was used, three years when a

correction was made for the growth of the litera-ture, and three years when a correction was made

for the growth of the SCI. The number of cita-tions received by an article peaked about two

years after its publication. In fact, when the datawere corrected for the growth of the literature orof the SCI, more citations were found in the first

year after publication than in any other year.The hypothesis that some pair among the three

methods would have statistically equivalent resultswas rejected for all pairwise comparisons when theMcNemar test for significance of change [8] wasapplied (Table 3). The results produced by the

T’able 3

Diachronous studv: McNemar test ’

’ tsX5o;= 28.869.NC = No correction. CGL = Corrected for growth of the

hterature. CGSCI = Corrected for growth of Sorenoe Crtunualudex.

Table 4

Diachronous study: relative rate of obsolescence (19 Years= 100 )

method in which the data were corrected for the

growth of SCI, and the method in which the datawere corrected for the growth of the literature

were similar (Table 4), but the null hypothesis wasrejected because of the great differences in the rateof decay for the first five years.

Because Line [4] claimed that a diachronous

study, corrected for the growth of SCI, is the

preferred method for measuring obsolescence, theother methods were compared to that ’standard’.When no corrections were made for growth, orwhen corrections were made for the growth of thehuman and medical genetics literature, the curvesproduced were different from the curve that

resulted when the standard technique was used:correcting for the growth of the literature pro-duced an inflated rate of obsolescence and

eliminating all corrections yielded a deflated rateof obsolescence.

In the synchronous study, publication dates forall journal articles listed in the references of the

199 sample articles were recorded. Of 3900 refer-ences examined, 3669 were published between 1961and 1979. References to articles published before1961 were excluded to make the periods dealt within the synchronous and diachronous studies equiv-alent. The synchronous situation was analysedwith no correction factor, with a correction for the

growth of the literature, and with a correction forthe growth of both literature and number of con-tributors.To calculate the correction for the growth of

the literature, the following equation was used:

.~, = corrected number of references in year i,

.v,, = number of references in year i,

j, = size of the literature in year i,12 591 - size of the literature in 1979.

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Fig. 5. Results from synchronous study.

To obtain the normalized number of references

(corrected for the growth of the literature and

contributors), the following equation was used:

x, =.Yz,118 231,

x, = corrected number of references in year i,

y, = number of references in year i, corrected for

the growth of the literature,z, = number of contributors in year i,

18 231 - number of contributors in 1979.

As with the diachronous study, the results of thesynchronous method were adjusted to a scale of100 (Fig. 5).

All three methods of analysing the data revealthat articles that are no more than two years old

Table 5

Synchronous studB: relative rate of obsolescence (19 Years

= 100 )

Tahle 6

S&dquo;nL&dquo;hr(Bn&dquo;lI~ tL]d’, McNemar test

a lRX~05 = 28.869.NC = No correction; CGL = Corrected for growth of theliterature; CGLC = Corrected for growth of the literature andcontnbutors.

have the greatest chance of being cited. A steadydecrease in the probability of being cited takes

place after articles are two years old.When the correction for growth of the literature

was applied, the probability that literature pub-lished before 1964 will be cited was observed to

increase. This did not occur when the other syn-chronous methods were used and it is inconsistentwith findings from other studies, suggesting thatcorrecting a synchronous study for the growth ofthe literature may distort the picture of obsoles-cence.

The rates of obsolescence found in the synchro-nous study are given in Table 5. The median

citation age was four years for the uncorrected

method, six years for the method that corrected

for the growth of the literature, and four years forthe method that corrected for the growth of bothliterature and contributors.

The results of the McNemar test applied to thesynchronous study are given in Table 6. The ratesof obsolescence were found to be statisticallyequivalent when x was the uncorrected data andy was the synchronous study in which a correctionwas made for the growth of the literature and

contributors. There seems to be no reason, there-

fore, for corrections to be made for the growth ofthe literature or the growth in contributors in a

synchronous study. That is, the two factors canceleach other out, as Brookes had suggested theywould.

Each diachronous study was compared with

each synchronous study method to see if the re-

sults from the synchronous studies were statisti-

cally equivalent to those from the diachronous

study in which a correction for the growth of theSCI was made (Table 7). In all cases, the null

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Table 7

McNemar test to compare synchronous and diachronous meth-ods

18xo 2 0, = 28.869.NC = No correction; CGL = Corrected for growth of the

literature, CGLC = Corrected for growth of the literature andcontributors. CGSCI = Corrected for growth of Screrrce Crtu-trnn Index.

hypothesis that the curves were equivalent wasrejected at the a = 0.05 level.

It was found that the rate of citation decreased

immediately in the diachronous study when a

correction was made for growth of the SCI,whereas, in the synchronous study, the number ofreferences increased for the first two years (paperspublished in 1979 cited 107 sources published in1979, 448 published in 1978, and 529 published in1977). Therefore, an analysis was conducted in

which the data for the first two years in the

synchronous study (i.e., 1979 and 1978) were ex-cluded and the data for 1960 were added). Thediachronous citation decay rate from 1961 to 1978was compared with the synchronous reference de-cay rate from 1977 to 1960. The justification forthis was that, since SCI was not available until1961, citations in articles published in 1960 werenot included in the diachronous study, likewise,the synchronous data for late 1978 and 1979 mayhave been incomplete (the literature of 1978-9would not be readily accessible to authors prepar-ing papers to be published in 1979). When data atone end of the curve were excluded, the number ofreferences and the number of citations had to be

Fig. 6. Results for 1961-1978 Diachronous study comparedwith 1977-1960 synchronous study without correction.

readjusted so that 0 again represented the lowestnumber of citations or references and 100 repre-sented the largest number of citations or refer-

ences.

Table X

McNemar test on adjusted data .

.. 17X~u:; = ~7.5~7.NC = No correction; CGL = Corrected for growth of theliterature; CGLC = Corrected for growth of the literature andcontnbutors; CGSCI = Corrected for growth of Smenre Cita-tion Index.

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Fig. 7. Results for 1961-197H diachronous study comparedwith 1977-1960 synchronous study corrected for hterature

growth and growth in number of contnbutors.

When the McNemar test was used in this anal-

ysis (Table 8), the null hypothesis was not rejected(at the a = 0.05 level) in two cases: when thediachronous data, corrected for the growth of theSCI, were compared with the uncorrected syn-chronous data; and when the diachronous data,corrected for the growth of the SCI, were com-pared with the synchronous data corrected for thegrowth of both literature and contributors (Figs. 6and 7).

Conclusions ’

In this investigation, the date of publication for13 734 citations in the diachronous study was

compared with the date of publication for 3669references in the synchronous study. The samplesize was adequate and the time span, 19 years, waslong enough to determine a rate of obsolescence.

The results of the diachronous study for the

years 1961-1979 were significantly different (a =

0.05) from the results of the synchronous study forthe same time period. The diachronous study curvetended to be smoother overall than the curve for

the synchronous study, but a more notable dif-

ference between the two curves appeared duringthe first two years. The synchronous study showed

I

an increase in the number of references during thistime, whereas the diachronous study showed a

steady decrease in the number of citations. at leastwhen the data were adjusted for the growth of theSCI. This was somewhat unexpected. It is not

known whether this phenomenon will also occurin other subject areas, when data are corrected forgrowth of the SCl, or if it is a peculiarity of thesubject used in the present study.When the first two years of the synchronous

data were excluded, it was found that the rate ofobsolescence measured diachronously and cor-

rected for the growth of SCI was statisticallyequivalent to that found in the uncorrected syn-chronous study. Although the diachronous methodmay seem more ’correct’, the easier synchronousmethod yields equivalent results, at least in thefield of human and medical genetics. The datacollected in this study suggest that the synchro-nous method gives an accurate measure of declinein use with age and that there is no justificationfor introducing any correction factor in the syn-chronous method.

It must be recognized, of course, that the re-

sults of this study apply only to citation data. Acomparison of synchronous and diachronousmethods based on use of materials in a librarymay give different results. Moreover, the fact thata correction for literature growth may not be

needed in a citation study does not necessarilyimply that an equivalent correction (for shelf spaceoccupied by volumes of various ages) will not beneeded in a study of declining use of librarymaterials with age. Conflicting evidence alreadyexists on this point (see, for example, Sandison[11] and Sullivan et al. [12])

References

[1] Bertram C. Brookes, The growth, utility, and obsolescenceof scientific periodical literature, Journal of Documenta-tion 26 (1970) 283-294.

[2] E.M. Ferraz Barboza, Estudo comparativo dos metodosusados para medir a obsolescencia da literatura cientifica,Master’s thesis, Instituto Brasileiro de Informação emCiencia e Technologia, 1978.

[3] Belver C. Griffith, Patricia N. Servi, Anita Anker and M.Carl Drott, The aging of scientific literature: a citation

analysis, Journal of Documentation 35 (1979) 179-196.[4] Maurice B. Line, Does physics literature obsolesce? A

study of variation of citation frequency with time forindividual journal articles in physics, BLL Review 2(1974)84-91.

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74

[5] Maurice B. Line and Brenda Carter, Change in the use ofsociological articles with time: a comparison of di-

achronous and synchronous data, BLL Review 2 (1974)124-129.

[6] Maurice B. Line and Alexander Sandison, ’Obsolescence’and changes in the use of literature with time, Journal ofDocumentation 30 (1974) 283-350.

[7] Maurice B. Line, Alexander Sandison and J. McGregor,Patterns of Citations to Articles within Journals: A Pre-

liminary Test of Scatter, Concentration, and Obsoles-

cence, Bath University Library, 1972, ERIC No. ED 076197.

[8] Q. McNemar, Psychological Statistics (Wiley, New York,4th ed., 1969).

[9] M.R. Oliver, The effect of growth on the obsolescence ofsemiconductor physics literature, Journal of Documenta-tion 27 (1971) 11-17.

[10] Mark Oromaner, Career contingencies and the fate of

sociological research, Social Science Information 12 (1973)97-111.

[11] A. Sandison, Densities of use, and absence of obsoles-cence, in physics journals at M.I.T., J. Amer. Soc. Inform.Sci. 25 (1974) 172-182.

[12] M.W. Sullivan et al., Obsolescence in biomedical journals:not an artifact of literature growth, Library Research 2(1980-1981) 29-46.

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