How Long Should the Introduction of a Research Paper Be? Data from 61,518 Examples

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:

	Introduction Length
	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:

Variables	Coefficient	Standard error	p-value
(Intercept)	6.450	<0.001	<0.001
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.

References

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.

Here’s a summary of the key findings

Overall length of the introduction section

Length of the introduction for different article types

Length of the introduction in different journals

References

Further reading