STATISTICAL PROPERTIES OF THE WIKIGRAPH

A.Capocci, V. Servedio, F. Colaiori,D. Donato, L.S. Buriol, S. Leonardi , GC

Centro “E. Fermi”

arXiv:physics/0602026

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

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STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

A Nature investigation aimed to find if Wikipedia is an authoritative source of information with respect to established sources as Encyclopedia Britannica.Among 42 entries tested, the difference in accuracy was not particularly great: • the average science entry in Wikipedia contained around four inaccuracies; • the one in Britannica, about three. On the other hand the articles on Wikipedia are longer on average than those of Britannica. This accounts for a lower rate of errors in Wikipedia.

STATISTICAL PROPERTIES OF THE WIKIGRAPH

(Nature 438, 900-901; 2005)

In a survey of more than 1,000 Nature authors • 70% had heard of Wikipedia of those

• 17% of those consulted it on a weekly basis.• less than 10% help to update it

• Introduction

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

Actually, things are a little bit more complicated

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

There is not only “control” by users, but also conflict of interests. Thereby sometimes is not possible to modify 100% of the structure since some sites are locked.One of the biggest scandal was the biography of Journalist John Seigenthaler who was accused to be involved in the murder of President J.F. Kennedy

Some issues and languages have more controls than others. An experiment made by Italian newspaper “L’espresso” introduced Deliberately some errors in two voices • One in the career of Football player Rui Costa (to be part of an Italian team in the early 90’s)• To introduce a non-existing philosopher

Obviously: • The error for the football player was corrected after 30’• The philosopher remained in place until the experiment was published ( at least two weeks)

• sociological reasons: the encyclopedia collects pages written by a number of indipendent and eterogeneous individuals. Each of them autonomously decides about the content of the articles with the only constraint of a prefixed layout. The autonomy is a common feature of the content creation in the Web. The wikipedia authors’ community is formed by members whose only wish is to make available to the world concepts and topics that they consider meaningful. In some sense, tracing the evolution of the wikipedia subsets should mirror the develop of significant trends within each linguistic community.

• generation on time: wikipedia provides time information associated with nodes. Moreover, it provides old information: time information for the creation and the modifications for each page on the dataset.

• independency of external links: wikipedia articles link mainly to articles on the same dataset.

• variety of graph sizes: it can be collected one graph by language, and the graph dimensions vary from a few hundred pages up to half million pages.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

WHY STUDYING WIKIPEDIA?

Summarizing:

• We have available all the history of growth, so that we can study the evolution

• We have an example of a “social” network of huge size

• We can compare the system produced by users of different language, thereby measuring the effect of different cultures.

• We can study Wikipedia as a case study for the World Wide Web

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Introduction

WE RECOVER A PREFERENTIAL ATTACHMENT MECHANISM FROM THE DATA.

DIFFERENT LANGUAGES PRODUCE SIMILAR STRUCTURES

WE FIND A SYSTEM SIMILAR TO THE WWW EVEN IF THE MICROSCOPIC RULE OF GROWTH IS VERY DIFFERENT.

The datasets of each language are available in two selfextracting files for mysql database. The table cur contains the current on-line articles, whereas the table old contains all previous versions of each current article. Old versions of an article are identified for using the same title, and not the same id. The dataset dumps are updated almost weekly, so the current graph is usually not more than a week old.

For generating a graph from the link structure of a dataset, each article is considered a node and each hyperlink between articles is a link in this graph. In the wikipedia datasets, each webpage is a single article. An article also might contain some external links that point pages outside the dataset. Usually wikipedia articles has no external links, or just a few of them. These kind of links are not considered for generating the wikigraphs, since we want to restrict the graph to pages into the set being analyzed.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Data

We generated six wikigraphs, wikiEN, wikiDE, wikiFR, wikiES, wikiIT and wikiPT, generated from the English, German, French, Spanish, Italian and Portuguese datasets, respectively. The graphs were obtained from an old dump of June 13, 2004. We are not using the current data due to disk space restrictions. The English dataset of June 2005 has more than 36 GB compacted, that is about 200 GB expanded.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Data

The page that was mostly visited was the main pages for wikiEN, wikiDE, wikiFR and wikiES, while that for the datasets wikiIT and wikiPT there were no visits associated with the pages.

• SCC (Strongly Connected Component) includes pages that are mutually reachable by traveling on the graph• IN component is the region from which one can reach SCC• OUT component encompasses the pages reached from SCC. • TENDRILS are pages reacheable from the IN component,and not pointing to SCC or OUT region TENDRILS also includes those pages that point to the OUT region not belonging to any of the other defined regions. • TUBES connect directly IN and OUT regions,• DISCONNECTED regions are those isolated from the rest.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Topology

The Bow-tie structure, found in the WWW (Broder et al. Comp. Net. 33, 309, 2000)

The percentage of the various components of the Wikigraph for the various languages.

STATISTICAL PROPERTIES OF THE WIKIGRAPH

The measure/size of the Wikigraph for the various languages.

• Topology

in–degree(empty) and out–degree(filled) Occurrency distributions for the Wikgraph

in English (○) and Portuguese ().

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Topology

The Degree shows fat tails that can be

approximated by a power-law function of

the kind P(k) ~ k-

Where the exponent isthe same both for in-degree and out-

degree.In the case of WWW

2 ≤ in ≤ 2.1

The average neighbors’ in–degree, computed along incoming edges, as a function of the in–degree for the English (○) and Portuguese ()

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Topology

As regards the assortativity (as measured by the

average degree of the neighbours of a vertex with degree k) there is

no evidence of any assortative behaviour.

The pagerank distribution for wikiEN is a power law function with γ = 2.1. Previous measures in webgraphs also exhibit the same behaviour for the pagerank distribution.

We list the number of visits of the top ranked pages just to show that this value is not related with the pagerank values. We confirm that very little correlation was found between the link analysis characteristics and the actual number of visits.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Topology

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Dynamics

Given the history of growth one can verify

the hypothesis of preferential

attachment. This is done by means of the histogram (k) who gives the number of

vertices (whose degree is k) acquiring new

connections at time t.This is quantity is

weighted by the factor N(t)/n(k,t)We find preferential

attachment for in and out degree.English (○) and Portuguese ().

White= in-degreeFilled = out-degree

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Dynamics

In our opinion the nature of this preferential attachment is effective ratther than the real driving force in the phenomenon.

In other words the linear preferential attachment can be originated by a copying procedure (new vertices are introduced by copying old ones and keeping most of the edges). Also we could have a sort of fitness for the various entries (but in this case one has a multidimensional series of quantities describing the importance of one page).

Apart the interpretation the data show a rather clear LINEAR PREFERENTIAL ATTACHMENT

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Dynamics

Other power-laws related to dyamics

need to be explainedFor example the

number of updates also follows a power law.

Each point presents the number of nodes (y axis) that were updated exactly x times.

STATISTICAL PROPERTIES OF THE WIKIGRAPH•Dynamics

This feature is time invariant

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Modelling

Actually 1) This network is oriented. 2) The preferential attachment in Wikipedia has a

somewhat different nature. Here, most of the times, the edges are added between existing vertices differently from the BA model. For instance, in the English version of Wikipedia a largely dominant fraction 0.883 of new edges is created between two existing pages, while a smaller fraction of edges points or leaves a newly added vertex (0.026 and 0.091 respectively).

From these data it seems that a model in the spirit of BA could reproduce most of the features of the system.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Modelling

We introduced an evolution rule, similar to other models ofrewiring already considered*,

• At each time step, a vertex is added to the network. It is connected to the existing vertices by M oriented edges; the direction of each edge is drawn at random:

•with probability R1 the edge leaves the new vertex pointing to an existing one chosen with probability proportional to its in–degree;• with probability R2, the edge points to the new vertex, and the source vertex is chosen with probability proportional to its out–degree.

• Finally, with probability R3 = 1 − R1 − R2 the edge is added between existing vertices: the source vertex is chosen with probability proportional to the out–degree, while the destination vertex is chosen with probability proportional to the in–degree.

* See for example Krapivsky Rodgers and Redner PRL 86 5401 (2001)

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Modelling

The model can be solved analytically P(kin) ~ kin

-inin1-R2))

P(kout) ~ kout

outout1-R1))

We can use for the model the empirical values of R1=0.026R2=0.091R3=0.083Already measured for the English version of Wikigraph

in out

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Modelling

The model can be solved analytically

Knnin

(kin) ~ M N1-R1 R1R2/R3 (R3≠0)

Knnin

(kin) ~ M R1R2 ln (N) (R3=0)

Both cases is constant

The value of the constant depends also upon the initial conditions. The two lines refer to two realizations of the model where in one case the 0.5% of the first vertices has been removed.

STATISTICAL PROPERTIES OF THE WIKIGRAPH• Conclusions

• We have a structure that resembles the bow-tie of the WWW

• We have a power-law decay for the degree distributions and also a power-law decay for the number of one page updates

• Preferential Attachment in the Rewiring seems to be the driving force in the evolution of the system

• The microscopic structure of rewiring is very different from that of WWW In principle a user can change any series of edges and add as many pages as wanted. Still most of the quantities are similar

It turns out that the pagerank of the pages is not related with the number of visit opens a very interesting scenario for further research work. Since, by definition, pagerank should give us the visit time of the page and since actually it is complety indipendent by the number of visits, we wonder if pagerank is a good measure of the authoritativeness of the pages in wikigraphs and which modifications should be introduced in order to tune its performances.

STATISTICAL PROPERTIES OF THE WIKIGRAPH•Conclusions