What Is Impact Factor?

The Basic Definition

The Journal Impact Factor (JIF) is a metric published annually by Clarivate (formerly Thomson Reuters) in the Journal Citation Reports. It measures how frequently the average article published in a journal was cited over the preceding two years.

In simple terms: a journal with an Impact Factor of 10 means that articles it published in the two previous years were cited, on average, 10 times each during the measurement year.

How Impact Factor Is Calculated

The formula is straightforward:

For example, if the New England Journal of Medicine published 500 citable articles in 2022–2023, and those articles received 30,000 citations in 2024, the 2024 Impact Factor would be 30,000 ÷ 500 = 60.0.

Only articles in journals indexed in Clarivate's Web of Science receive an Impact Factor. Journals not indexed — including many legitimate, high-quality publications — have no IF at all.

Impact Factor by Field: Why Comparison Across Disciplines Is Meaningless

Citation cultures differ dramatically by field. Biomedical journals accumulate citations at a rate that basic science, mathematics, or humanities journals simply cannot match — not because the science is better, but because biomedical papers typically cite many more references, and the research community is larger.

Comparing an oncology journal (IF 40) to a mathematics journal (IF 4) tells you nothing about the relative quality of their science. Always evaluate Impact Factor within a specific field.

Key Limitations You Must Know

1. Reviews inflate the IF

Review articles attract vastly more citations than original research papers, because they serve as reference points for many subsequent papers. Journals that publish many reviews will naturally have higher Impact Factors — which doesn't mean their original research is superior.

2. The two-year window is arbitrary

Basic science research, clinical trials, and slow-maturing fields accumulate citations over 5–10 years. A two-year citation window systematically undervalues journals in those fields relative to fast-moving clinical subspecialties.

3. Self-citation manipulation

Journals can game their IF by encouraging authors to cite articles published in the same journal. Clarivate suppresses Impact Factors of journals caught doing this, but the incentive remains.

4. It measures the journal, not the article

A paper published in the NEJM may receive zero citations, while a paper in a lower-IF journal may become a landmark study. The IF tells you about the journal's average — it says nothing about the specific paper you are reading.

Impact Factor vs. SJR vs. CiteScore

Because of these limitations, several alternatives have emerged:

• SJR (SCImago Journal Rank) — weights citations by the prestige of the citing journal, not just their count. A citation from Nature counts more than one from a low-ranked journal. Freely available, covers more journals than Clarivate's IF. Learn more about SJR →
• CiteScore (Elsevier/Scopus) — uses a 4-year window and counts all document types, not just "citable items." Freely available and more inclusive than IF.
• H5-index (Google Scholar) — based on the highest number h such that h articles published in the last 5 years have at least h citations each.

What Impact Factor Should Mean for Your Literature Search

When searching PubMed, a paper's journal Impact Factor is one quality signal among several. A result published in NEJM (IF ~96) or Lancet (IF ~98) has passed an exceptionally rigorous review process. But do not discard a useful paper from a specialized journal simply because its IF is lower — a niche oncology journal with IF 8 may be the field's leading outlet.

PubRank addresses this by ranking PubMed results using SJR scores within each discipline, allowing you to see the relative quality of each source in context.