**3.1. Collaboration in the category demography**

Concurrently with the increase of the production of documents along the period between 2000 and 2016, there was also an increase in the number of authors per article and, with it, collaboration in the category *Demography* (**Figure 1**). There is a correlation of 0.992 with a significance of 0.01, between the number of published documents and the number of documents with multiple authorships. Early in the set period of time, the difference between single and multiple authorship documents was of only 8.8%. Despite continuous ups and downs, such difference increased slowly by up to 20% in 2009. In 2010, the difference increased to 42% and remained ever since within an interval of a minimum of 30.3% to a maximum of 51.7%.

All this multiple authorship has an impact on collaboration indexes. In this line, the DC increased gradually from 0.52 in year 2000 to a top 0.67 in 2014 and 2015. Likewise, the CI ranges between an initial 1.87 and a maximum of 2.34 in 2014 (**Table 1**). The overall values for the time interval between years 2000 and 2016 are DC = 0.605 and CI = 2.14.

**Figure 2** shows that the 70.4% of the documents from the *Demography* category are signed by one or two authors. A total of 39.5% of the papers have only one author, while articles with four or less authors only represented 13.07%.

The production of documents within the category *Demography* between 2000 and 2016 received a total of 147,024 citations. The average citation is of 12.9 cites per document (*SD* = 32.54), notwithstanding that 1840 received no citation at all, which represents 16.25 of total production.

Analysing citation in relation with author collaboration, it can be seen that multi-author documents receive 63.98% of the total citations while single-author documents receive 36.02%. In differentiating documents according to the number of authors, the highest citation is received by the documents signed by a single author, followed by those signed by two and three

**Figure 1.** Diachronic type of authorship in the category demography.


authors, and decreases as the number of signatory authors increases (**Figure 3**). There is a moderated correlation between citation and number of signatory authors, with a Pearson's correlation coefficient of 0.709 (*p* = .001) for the category. In the same line, citation and DC present a correlation coefficient of 0.542 (*p* = .025). Eight articles received more than 300 cita-

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Regarding international collaboration, only 10,479 documents (out of 11,361) presented affiliation information. The documents of the category *Demography* were written by authors affiliated to institutions of 147 different countries. Most of the documents (77.6%) in the sample are written by authors from the same country regardless if they are written by multiple authors

tions; one article received 806 citations.

**Figure 3.** Citation by number of authors in demography 2000–2016.

**Figure 2.** Co-authored distribution in demography (2000–2016).

**Table 1.** Degree of collaboration and collaboration index in the category demography.

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**Figure 2.** Co-authored distribution in demography (2000–2016).

**Figure 1.** Diachronic type of authorship in the category demography.

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**Year DC CI** 0.52 1.87 0.54 1.90 0.58 2.03 0.54 2.01 0.55 2.03 0.57 2.09 0.54 1.96 0.58 2.02 0.56 2.01 0.56 2.05 0.63 2.16 0.59 2.11 0.62 2.17 0.64 2.28 0.67 2.32 0.67 2.34 0.52 1.87

**Table 1.** Degree of collaboration and collaboration index in the category demography.

**Figure 3.** Citation by number of authors in demography 2000–2016.

authors, and decreases as the number of signatory authors increases (**Figure 3**). There is a moderated correlation between citation and number of signatory authors, with a Pearson's correlation coefficient of 0.709 (*p* = .001) for the category. In the same line, citation and DC present a correlation coefficient of 0.542 (*p* = .025). Eight articles received more than 300 citations; one article received 806 citations.

Regarding international collaboration, only 10,479 documents (out of 11,361) presented affiliation information. The documents of the category *Demography* were written by authors affiliated to institutions of 147 different countries. Most of the documents (77.6%) in the sample are written by authors from the same country regardless if they are written by multiple authors or not. **Figure 4** shows a tendency in the increase of the international collaboration between authors which provide much better visibility and further citation to the work [13, 15].

There are only 11 countries (*Barbados, Bolivia, Cape Verde, Hong Kong, Malta, Oman, Solomon Islands, Syria, Trinidad and Tobago, Yemen, Yugoslavia*) that do not collaborate with other countries in the sample. The country with most co-authorship with other countries in the world is the *USA*, relating with 104 countries. A total of 50.34% of the countries (74) have relationships with a maximum of four other countries.

*France, Germany, England* and the *USA* are the only four countries that co-write articles with more than 50 other countries.

The network depicted in **Figure 5** shows a general view over the country network for Demography considering all the period. Every vertex represents a country; the volume of a vertex is proportional to the number of documents written by authors of the country. The lines between vertices show that the linked countries co-write documents and the colour of the lines are proportional to the number of documents shared. International collaboration networks tend to be very dense. The density of the network in **Figure 5** is 0.06737933 which indicated that the network is dense for social sciences. The average degree of the countries is 9.9048, which means that each of the 147 countries in the network shares documents with almost 10 other countries.

There are 728 collaborations detected, most of them being anecdotal; 48.08% of these collaborations appear only once, which means that these two countries only co-write one document in the whole period. The most prolific relationships among countries are found to be between *England* and the *USA, Canada* and the *USA*, *Germany* and the *USA* with more than 100 documents shared by each.

**3.2. Collaboration in the category urban studies**

**Figure 5.** General network for international collaboration in demography 2000–2016.

interval analysed, the value of DC = 0.813 and CI = 2.07.

with four or less authors only represented 10.88%.

a positive significance (.926, p < .01).

the documents have never been cited.

In this category, the documents ranged from 1015 in the year 2000 to 2170 in 2016, so that production has doubled since the beginning of the period studied (**Figure 6**). Initially, the percentage of documents signed by only one author (54.58%) was slightly higher than the one for multi-authored documents (45.42%). These values have varied along the years, with the proportion being reversed in 2016, reaching 68.89% for multi-authorship and 31.11% for single authorship. Since 2005, the number of multi-authored documents prevails, showing a continued growth. The average of authors is 2.06 authors per document (*SD* = 1.27). There is a correlation between the total production of the documents and those of multi-authorship with

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Indicators suggest that this collaboration has increased in the period. The DC increased from 0.45 to 0.69, while the CI varied from 1.67 in 2000 to 2.43 in 2016 (**Table 2**). Globally for the

**Figure 7** shows that the 89.12% of the documents from the *Demography* category are signed by one, two or three authors. A total of 40.89% of the papers have only one author, while articles

Between 2000 and 2016, the *Urban Studies* category received 377,473 citations. The average is that every document in the sample has been cited 15.6 times (*SD* = 29.27). A total of 11.14% of

The multi-authored documents received 61.8% of the citations, while those written by a single author received 38.2%. According to the number of authors, the highest citation is received by papers signed by a single author, followed by those of two and three authors, all of whom

**Figure 4.** Diachronic international collaboration in demography 2000–2016.

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**Figure 5.** General network for international collaboration in demography 2000–2016.

### **3.2. Collaboration in the category urban studies**

or not. **Figure 4** shows a tendency in the increase of the international collaboration between

There are only 11 countries (*Barbados, Bolivia, Cape Verde, Hong Kong, Malta, Oman, Solomon Islands, Syria, Trinidad and Tobago, Yemen, Yugoslavia*) that do not collaborate with other countries in the sample. The country with most co-authorship with other countries in the world is the *USA*, relating with 104 countries. A total of 50.34% of the countries (74) have relationships

*France, Germany, England* and the *USA* are the only four countries that co-write articles with

The network depicted in **Figure 5** shows a general view over the country network for Demography considering all the period. Every vertex represents a country; the volume of a vertex is proportional to the number of documents written by authors of the country. The lines between vertices show that the linked countries co-write documents and the colour of the lines are proportional to the number of documents shared. International collaboration networks tend to be very dense. The density of the network in **Figure 5** is 0.06737933 which indicated that the network is dense for social sciences. The average degree of the countries is 9.9048, which means that each of the 147 countries in the network shares documents with

There are 728 collaborations detected, most of them being anecdotal; 48.08% of these collaborations appear only once, which means that these two countries only co-write one document in the whole period. The most prolific relationships among countries are found to be between *England* and the *USA, Canada* and the *USA*, *Germany* and the *USA* with more than 100 docu-

authors which provide much better visibility and further citation to the work [13, 15].

with a maximum of four other countries.

more than 50 other countries.

72 Scientometrics

almost 10 other countries.

ments shared by each.

**Figure 4.** Diachronic international collaboration in demography 2000–2016.

In this category, the documents ranged from 1015 in the year 2000 to 2170 in 2016, so that production has doubled since the beginning of the period studied (**Figure 6**). Initially, the percentage of documents signed by only one author (54.58%) was slightly higher than the one for multi-authored documents (45.42%). These values have varied along the years, with the proportion being reversed in 2016, reaching 68.89% for multi-authorship and 31.11% for single authorship. Since 2005, the number of multi-authored documents prevails, showing a continued growth. The average of authors is 2.06 authors per document (*SD* = 1.27). There is a correlation between the total production of the documents and those of multi-authorship with a positive significance (.926, p < .01).

Indicators suggest that this collaboration has increased in the period. The DC increased from 0.45 to 0.69, while the CI varied from 1.67 in 2000 to 2.43 in 2016 (**Table 2**). Globally for the interval analysed, the value of DC = 0.813 and CI = 2.07.

**Figure 7** shows that the 89.12% of the documents from the *Demography* category are signed by one, two or three authors. A total of 40.89% of the papers have only one author, while articles with four or less authors only represented 10.88%.

Between 2000 and 2016, the *Urban Studies* category received 377,473 citations. The average is that every document in the sample has been cited 15.6 times (*SD* = 29.27). A total of 11.14% of the documents have never been cited.

The multi-authored documents received 61.8% of the citations, while those written by a single author received 38.2%. According to the number of authors, the highest citation is received by papers signed by a single author, followed by those of two and three authors, all of whom

received 88.31% of the citations (**Figure 8**). Data present a high correlation between citation and number of signatory authors, with a Pearson's correlation coefficient of 0.892 (*p* = .00) for the category *Urban Studies*. Citation-DC correlation coefficient was 0.878 (*p* = .00) which is an evidence of strong correlation between both variables. It is evident that for documents signed by more than four authors there is a decrease in the number of citations received. Seven arti-

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For the international collaboration, only 23,577 registers were considered for being the only ones that incorporate information about authors' affiliation. The authors were affiliated to

The documents of the category *Urban Studies* were mostly written by authors affiliated to the same country, in fact only 16.12% of the documents were written in international collaboration. **Figure 9** shows these results analysing the international collaboration along the period. We can see an increase in this collaboration since 2010, resulting in this tendency being slightly lower than the one found for *Demography* sample. It is remarkable that the category *Urban Studies* involved less countries than *Demography* which led to a less collaboration

Only 9 countries out of 133 contributed with documents without international collaboration (*Algeria, Azerbaijan, Bolivia, Hong Kong, Morocco, Sudan, Tunisia, Uruguay, Yugoslavia*) that do

The most cited document has 2004 citations and is signed by 2 authors.

cles received more than 500 citations.

**Figure 7.** Co-authored distribution in urban studies (2000–2016).

institutions of 133 countries.

among countries.

**Figure 6.** Diachronic type of authorship in the category urban studies.


**Table 2.** Degree of collaboration and collaboration index in the category urban studies.

received 88.31% of the citations (**Figure 8**). Data present a high correlation between citation and number of signatory authors, with a Pearson's correlation coefficient of 0.892 (*p* = .00) for the category *Urban Studies*. Citation-DC correlation coefficient was 0.878 (*p* = .00) which is an evidence of strong correlation between both variables. It is evident that for documents signed by more than four authors there is a decrease in the number of citations received. Seven articles received more than 500 citations.

The most cited document has 2004 citations and is signed by 2 authors.

For the international collaboration, only 23,577 registers were considered for being the only ones that incorporate information about authors' affiliation. The authors were affiliated to institutions of 133 countries.

The documents of the category *Urban Studies* were mostly written by authors affiliated to the same country, in fact only 16.12% of the documents were written in international collaboration. **Figure 9** shows these results analysing the international collaboration along the period. We can see an increase in this collaboration since 2010, resulting in this tendency being slightly lower than the one found for *Demography* sample. It is remarkable that the category *Urban Studies* involved less countries than *Demography* which led to a less collaboration among countries.

Only 9 countries out of 133 contributed with documents without international collaboration (*Algeria, Azerbaijan, Bolivia, Hong Kong, Morocco, Sudan, Tunisia, Uruguay, Yugoslavia*) that do

**Figure 7.** Co-authored distribution in urban studies (2000–2016).

**Figure 6.** Diachronic type of authorship in the category urban studies.

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**Year DC CI** 0.45 1.67 0.45 1.68 0.44 1.69 0.50 1.76 0.46 1.75 0.53 1.86 0.52 1.86 0.56 1.97 0.55 1.91 0.60 2.04 0.62 2.09 0.65 2.16 0.65 2.19 0.64 2.21 0.67 2.31 0.68 2.40 0.69 2.43

**Table 2.** Degree of collaboration and collaboration index in the category urban studies.

There are 861 collaborations detected, most of them being anecdotal; 44.83% of these collaborations appear only once which means that these 2 countries only co-write 1 document in the whole period. The most prolific relationships among countries are found to be between *People's Republic of China* and the *USA, Canada* and the *USA* and *England* and the *USA* with

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Comparing the number of journals indexed in JCR for the two categories analysed, it can be seen that *Demography* accounts for 68.4% of the number of journals for *Urban Studies* and its production only represents 47.36% of the second. In both categories, the percentages of multiauthored documents have very similar values with minor differences around l% (**Table 3**).

Throughout 2000 and 2016, the citation in *Urban Studies* has been increasing with an exponential behaviour (R2 = 0.9712) as well as the number of multi-authored articles (R2 = 0.8214). However, the category *Demography* behaves differently, the increase in citations has a logarithmic behaviour (R2 = 0577) and the number of articles written in collaboration represents a linear model (R2 = 0.9557) (**Figure 11**). This relationship between the citations received and the number of multi-authored documents in the two categories (*Urban studies: Pearson's coeff.* 0.892, *p* = .00; *Demography: Pearson's coeff.* = 0.709, *p* = .001) is in agreement with that found in other studies in which it has been shown that co-authorship has a tangible effect on the impact

It is remarkable that, for *Demography*, DC values have always been higher than 0.5; in addition, DC values are similar to those obtained for some other research fields of social sciences such as *basic psychology* between 1926 and 2005 [43]. There is a linear dependency between

**Figure 10.** General network for international collaboration in urban studies 2000–2016.

**3.3. Comparison between the categories demography and urban studies**

more than 100 documents shared by each.

of the citations [41, 42].

**Figure 8.** Citation by number of authors in urban studies 2000–2016.

**Figure 9.** Diachronic international collaboration in urban studies 2000–2016.

not collaborate with other countries in the sample. The country with most co-authorship with other countries in the world is the *USA*, collaborating with 81 countries. A total of 49.62% of the countries (66) have relationships with a maximum of 5 other countries.

*France, Canada, the Netherlands, England and* the *USA* are the countries collaborating with more than 50 other countries in the category.

The network depicted in **Figure 10** shows a general view over the country network for *Urban Studies* considering all the period. The density of the network is 0.09808612, higher than the one found for *Demography* which also indicated that the network is dense for social sciences. The average degree of the countries is 12.9473.

There are 861 collaborations detected, most of them being anecdotal; 44.83% of these collaborations appear only once which means that these 2 countries only co-write 1 document in the whole period. The most prolific relationships among countries are found to be between *People's Republic of China* and the *USA, Canada* and the *USA* and *England* and the *USA* with more than 100 documents shared by each.
