I analyzed a random sample of 61,518 full-text research papers, uploaded to PubMed Central between the years 2016 and 2021, in order to answer the questions:
What is the typical length of an introduction section? and which factors influence it?
I used the BioC API to download the data (see the References section below).
Here’s a summary of the key findings
1. The median introduction was 553 words long (equivalent to 21 sentences, or 4 paragraphs), and 90% of the introductions were between 245 and 1,245 words.
2. The introduction is the shortest section of the research paper (14.6% of the total word count), approximately half the length of other sections (Methods, Results, and Discussion).
3. The length of the introduction does not differ between review articles and original research articles.
4. The quality of the journal does not influence the length of the introduction section.
Overall length of the introduction section
Here’s a table that describes the length of an introduction in terms of words, sentences, and paragraphs:
|Word Count||Sentence Count||Paragraph Count|
|Minimum||23 words||1 sentence||1 paragraph|
|25th Percentile||401 words||15 sentences||3 paragraphs|
|50th Percentile (Median)||553 words||21 sentences||4 paragraphs|
|Mean||632.4 words||23.7 sentences||4.6 paragraphs|
|75th Percentile||762 words||28 sentences||5 paragraphs|
|Maximum||9,776 words||426 sentences||196 paragraphs|
From these data, we can conclude that most research papers have a 3 to 5 paragraph introductions (consisting of 15 to 28 sentences).
If you are interested, here’s the link to the PubMed article that had the longest introduction of 9,776 words, 426 sentences, and 196 paragraphs!
The introduction constitutes 14.6% of the total word count of a research paper, nearly half the length of other sections (Methods, Results, and Discussion) [source: How Long Should a Research Paper Be?].
Length of the introduction for different article types
The following table shows the median word count of the introduction section for different study designs:
|Study design||Number of studies in the sample||Median introduction word count|
|Case series||140 studies||348 words|
|Case-control||443 studies||446 words|
|Cohort||5,181 studies||464 words|
|Meta-analysis||1,481 studies||465 words|
|Randomized controlled trial||842 studies||497 words|
|Cross-sectional||3,529 studies||524 words|
|Systematic review||689 studies||535 words|
|Pilot study||686 studies||570 words|
|Case report||407 studies||576 words|
|Quasi-experiment||144 studies||665 words|
The data show no clear pattern since the introductions of review articles and original research articles have almost similar word counts. So we can conclude that there is no particular article type that requires a longer introduction section.
Length of the introduction in different journals
In order to study the influence of the journal quality on the length of the introduction, I ran a Poisson regression that models the introduction word count given the journal impact factor. Here’s the model output:
|Journal impact factor||0.008||<0.001||<0.001|
The model shows that a higher journal impact factor is associated with a longer introduction. Although statistically significant, this result is practically negligible since an increase of 1 unit in the journal impact factor is associated with an increase of only 0.8% in the introduction word count.
- Comeau DC, Wei CH, Islamaj Doğan R, and Lu Z. PMC text mining subset in BioC: about 3 million full text articles and growing, Bioinformatics, btz070, 2019.
- How Long Should the Methods Section Be? Data from 61,514 Examples
- How Many References to Cite? Based on 96,685 Research Papers
- How Old Should References Be? Based on 3,823,919 Examples
- Length of a Conclusion Section: Analysis of 47,810 Examples
- Statistical Software Popularity in 40,582 Research Papers
- Programming Languages Popularity in 12,086 Research Papers