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 CountSentence CountParagraph Count
Minimum23 words1 sentence1 paragraph
25th Percentile401 words15 sentences3 paragraphs
50th Percentile (Median)553 words21 sentences4 paragraphs
Mean632.4 words23.7 sentences4.6 paragraphs
75th Percentile762 words28 sentences5 paragraphs
Maximum9,776 words426 sentences196 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 designNumber of studies in the sampleMedian introduction word count
Case series140 studies348 words
Case-control443 studies446 words
Cohort5,181 studies464 words
Meta-analysis1,481 studies465 words
Randomized controlled trial842 studies497 words
Cross-sectional3,529 studies524 words
Systematic review689 studies535 words
Pilot study686 studies570 words
Case report407 studies576 words
Quasi-experiment144 studies665 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:

VariablesCoefficientStandard errorp-value
Journal impact factor0.008<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.

Further reading