Citation Counts in PLoS Biology Articles

As in a number of earlier blog posts, I will play around with just over 1,700 PLoS Biology articles that I downloaded last year.

In this blog post I will attempt to tackle a question that I have been curious about for a long time now: How often are references cited in the main text of scientific articles? That is, what is the citation count of each reference listed in the bibliography of peer-reviewed articles?

This question touches upon the observation that many references cited in a given article are rather shallow, i.e. the citing the author may not cite certain references for their ingenious influence on their study but rather for any number of alternate reasons.

My current approach to this question is to use regular expressions in an attempt to detect as many citations in a given article as possible.

As I am rather unfamiliar with regular expressions and citation typography is not uniform across all articles in my data set I am certain that the citation counts I compute are imperfect.

Therefore, please remember that the results graphed at the bottom should be interpreted as approximations.

I would be extremely grateful for any improvements to my code that you may be able to contribute for better data collection. (Just get in touch if you would like to contribute)

To read the articles I have downloaded, I will use my own helper library.

import plospy
import os

from nltk.tokenize import sent_tokenize

from matplotlib import pyplot
%matplotlib inline

from nltk.tag import RegexpTagger, BigramTagger, UnigramTagger
from nltk.corpus import brown
import nltk

import re
from nltk import RegexpTokenizer

from collections import Counter

import numpy

all_names = [name for name in os.listdir('../plos/plos_biology/plos_biology_data') if '.dat' in name]

article_bodies = {}
article_titles = {}

for name_i, name in enumerate(all_names):
    docs = plospy.PlosXml('../plos/plos_biology/plos_biology_data/'+name)
    for article in
        article_bodies[article['id']] = article['body']
        article_titles[article['id']] = article['title']

Variable article_bodies now contains the main texts of the articles in our data set.

Below, I will use RegexpTokenizer instances to capture different styles of citations (in hindsight, I probably could have gotten away with just using re.findall instead of RegexpTokenizer).

The logic in the code below is that some articles use numeric citations such as [1] and some others use named citations such as Name et al. 2014. There, of course, variations of these such as ranges and series in the numeric style, e.g. [1 - 10] and [1,2,3,4] respectively, and single authors and author pairs in the named style, e.g. Name 2014 and Name and Nombre 2014 respectively.

The code snippet below attempts to take care of these variations to the best of my current regex mastery and available spare time.

tokenizers = [('single', RegexpTokenizer(r'\[\s*\d+\s*\]')),
              ('range', RegexpTokenizer(r'\[\s*(\d+)\s*(–|-|\u2013)\s*(\d+)\s*\]')),
              ('name-et-al-year', RegexpTokenizer(r'[a-zA-Z]+\set al.\s19\d{2}|[a-zA-Z]+\set al.\s20\d{2}')),
              ('name-year', RegexpTokenizer(r'[a-zA-Z]+\s19\d{2}|[a-zA-Z]+\s20\d{2}')),
              ('name-and-name-year', RegexpTokenizer(r'[a-zA-Z]+\sand\s[a-zA-Z]+\s19\d{2}|[a-zA-Z]+\sand\s[a-zA-Z]+\s20\d{2}')),
              ('series', RegexpTokenizer(r'\[\s*\d+\s*(,\s*\d+\s*)+\]'))]

citations = {}

for doi_i, doi in enumerate(article_bodies.keys()):
    body = article_bodies[doi]
    citations[doi] = {}
    citations[doi]['named'] = []
    citations[doi]['indexed'] = []
    for (name, tokenizer) in tokenizers:
            tokens = tokenizer.tokenize(body.encode('utf8'))
            for token in tokens:
                if name == 'single':
                    [index] = re.findall(r'\d+', token)
                    index = int(index)
                    if index < 1:
                elif name == 'name-year':
                    if 'al.' in token:
                    elif token[0] == ',':
                    elif token[:2].lower() == 'in':
                    elif token[0].lower() == 'a':
                elif name == 'name-et-al-year' or name == 'name-and-name-year':
                elif name == 'range':
                        dummy = re.findall(r'\d+', token)
                        start = dummy[0]
                        end = dummy[-1]
                        print 'doi_i', doi_i
                        print token
                    start = int(start)
                    end = int(end)
                    citations[doi]['indexed'] += [i for i in range(start, end+1)]
                elif name == 'series':
                    if token.strip() == '[0,1]':
                    indeces = re.findall(r'\d+', token)
                    citations[doi]['indexed'] += [int(index) for index in indeces]

for doi in citations:
    if len(citations[doi]['indexed']) > len(citations[doi]['named']):
        citations[doi]['list'] = citations[doi]['indexed']
        citations[doi]['type'] = 'indexed'
        start = min(citations[doi]['indexed'])
        end = max(citations[doi]['indexed'])
        citations[doi]['counter'] = Counter({i: 0 for i in range(start, end+1)})
        if start != 1 or len(citations[doi]['counter'].keys()) > 200:
            citations[doi]['ignore'] = True
            citations[doi]['ignore'] = False

        citations[doi]['list'] = citations[doi]['named']
        citations[doi]['type'] = 'named'
        citations[doi]['counter'] = Counter(citations[doi]['named'])
        if len(citations[doi]['counter'].keys()) > 200:
            citations[doi]['ignore'] = True
            citations[doi]['ignore'] = False

Some citations are rather hard to parse with regular expressions and so we will ignore those articles whose citation style appears to use numbered references and we were unable to detect reference 1. We will further ignore those articles that we think contain a ridiculous number of references (here, we will use a cutoff of 200 items in the bibliography). We should likely think about other ways that our parsing can fail.

Hence, we will ignore this many articles out of our total set of articles:

sum([1 for doi in citations if citations[doi]['ignore']])


Let us take a look at how many unique references (i.e. the number of items in the bibliography of the article) we detect for each article that we do not ignore:

pyplot.scatter([i for i, doi in enumerate(citations) if not citations[doi]['ignore']],
               [len(citations[doi]['counter'].keys()) for doi in citations if not citations[doi]['ignore']])

<matplotlib.collections.PathCollection at 0xb762c10>


Daniel Lemire and coauthors observed an average of 30 references cited per article.

To see if our citation counts make any sense, let us see what average we observe:

numpy.mean([len(citations[doi]['counter'].keys()) for doi in citations if not citations[doi]['ignore']])


The mean that we observe is about twice as big as that of Lemire et al. so this may either be a red flag for our data collection method or due to the difference in research fields.

Let us now take a look at the greatest number of times a reference is mentioned in each article:

pyplot.scatter([i for i, doi in enumerate(citations) if not citations[doi]['ignore']],
               [citations[doi]['counter'].most_common(1)[0][1] for doi in citations if not citations[doi]['ignore']])

<matplotlib.collections.PathCollection at 0xb624e50>


for doi in citations:
    if not citations[doi]['ignore'] and citations[doi]['counter'].most_common(1)[0][1] > 30:
        print doi
        print citations[doi]['counter'].most_common(1)


  • [(16, 39)]


  • [(29, 47)]


  • [(1, 38)]


  • [(13, 32)]


  • [(41, 32)]


  • [(1, 42)]


  • [(18, 34)]


  • [(17, 42)]

(Read the above output as [(reference number in article, number of times this reference is cited)])

Going through the above articles by hand, it is interesting to see that most of the counted citations are actually real: reference 13 in 10.1371/journal.pbio.1001115 is actually cited 32 times in the main text and once in the figure captions (the latter are not part of my data set).

However, reference 1 in 10.1371/journal.pbio.1000132 occurs only five times in the main text while the string [1] as part of the table in the specific format that I downloaded these articles in occurs many more times.

In the same article, 10.1371/journal.pbio.1000132, the editors seem to have made use of a different typographical scheme for indicating ranges of references as in this article we find ranges such as [1] - [3] as opposed to the far more common [1 - 3].

The gist of this digression is that my regular expressions above are not bulletproof and we will count some references too often and some other references not often enough.

With this caveat in mind, let us now take a look at the distribution of citation counts across all PLoS Biology articles that we do not ignore (see above).

counts = []
for doi in citations:
    if not citations[doi]['ignore']:
        counts += citations[doi]['counter'].values()

n, bins, patches = pyplot.hist(counts, normed=True, bins=range(1,max(counts)))


n, bins, patches = pyplot.hist(counts, normed=True, bins=range(1,10))


n, bins, patches = pyplot.hist(counts, normed=False, bins=range(1,10))


The bins in the above histograms are [1,2[, [2,3[, etc. (see this question) so that, by far, the greatest number of times references are cited is once.

However, as mentioned multiple times in this post, I need to go back and validate my method of data collection better to have greater trust in this result.

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