Plagiarism vs Similarity Score: What's the Difference?

Researchers frequently confuse similarity scores with plagiarism accusations. When iThenticate returns a 22% similarity score, many researchers panic — assuming they're being accused of academic misconduct. This misunderstanding leads to unnecessary anxiety and sometimes to over-editing that weakens the manuscript. Understanding the difference is essential for navigating the submission process confidently.

Similarity Score: A Mathematical Measurement

A similarity score is produced by text-matching software (iThenticate, Turnitin, Copyleaks). It measures the percentage of your manuscript text that appears in the software's database — including published papers, websites, preprints, and other sources. The software has no ability to determine intent, context, or whether attribution was provided. It simply measures overlap.

A 15% similarity score could result from:

• Properly cited quotations from published papers
• Standard academic phrases that appear in thousands of papers
• Your own previously published work (self-citation)
• Shared methodology descriptions for validated instruments
• Reference list formatting matches

Plagiarism: An Ethical Judgment

Plagiarism is the use of someone else's ideas, words, or work without appropriate attribution — presented as your own. It is an ethical violation, not a statistical measurement. Plagiarism can exist even when the similarity score is low: paraphrasing someone's argument without citation produces low text similarity but constitutes plagiarism.

The Four Types of Plagiarism in Academic Publishing

How Journal Editors Evaluate Similarity Reports

Editors do not simply look at the total similarity percentage. They examine the iThenticate report breakdown to assess:

• Source concentration: Is 15% similarity spread across 20 sources, or concentrated in one source (suspicious)?
• Match type: Are matches in the reference list (expected) or in the body text (concerning)?
• Match length: Short phrase matches (8–15 words) are different from paragraph-length matches
• Attribution: Are the matching passages properly cited?

Self-Plagiarism: The Most Common Form

Self-plagiarism — reusing substantial portions of your own previously published work without disclosure — is the most common form of plagiarism detected in Q1 journal submissions. The most vulnerable sections are methodology (researchers often copy their own validated instruments verbatim) and literature review (reusing background sections from prior papers).

Most journals permit self-citation of prior work if: the prior work is cited, the reused section is clearly identified, and the new paper makes a substantially new contribution. Check your specific journal's policy on prior publication before submission.

What to Do If Your Similarity Score Is High

1. Download the full iThenticate report and review each match source by source
2. Identify matches in body text vs references — reference matches are usually fine
3. For body text matches: check whether each match is properly cited
4. Rewrite uncited matching passages in your own words with proper attribution
5. For self-plagiarism: add disclosure of prior publication or substantially rewrite
6. Re-run the check before submission

Conclusion

Similarity score and plagiarism are related but distinct concepts. A high similarity score requires investigation, not panic. A low similarity score does not guarantee ethical writing. Focus on proper attribution, original expression of ideas, and transparent disclosure of prior work — these are the standards editors actually apply.