The similarity index shows text or phrases used in another published work. In your case somehow, you must have used similar text from another paper with minor changes in the sentences but the word by word arrangement looks similar. For instance, if you used Scribbr which detect unintentional plagialism, it could be even higher than what you found in turnitin. I understand it is a little difficult to resolve this because of specific technical terms used in STEM, which makes the removal of plagialism ultimately change the proposition intended. Some journals may not accept this level of plagialism because it may be considered the worst form of direct plagialism if the editors do not read your manuscript word by word. Try to reanalyze, rearrange and paraphrase your sentences and avoid specific word by word phrases which contribute to this similarity index. Make sure you only use terms from another source that will affect the content if removed from the paper.

I suggest finding the sentences that hit plagiarism. You may use your paper's checker or plagiarism checkers. There are many free softwares for this. You can install one of them to your browser and it automatically check it. Dupli was one of those that were well known. I believe Grammarly also has plagiarism-checker:

https://www.grammarly.com/plagiarism-checker

To reduce it naturally, you should use your own words. This must be always. If you still hit it, then you could rephrase to a similar phrase. Change the wordings and change the order of your words. You can check a paragraph one by one until you eliminate it.

Thanku Job Omweno and Shin Murakami for your answers. These were helpful.

Is your paper presenting an original idea / contribution? If so, then cite that prior paper (with 89% similar words) and focus more words in your paper to highlighting your novel findings.

Hi Pankaj Verma ,

As you enquired about the similarity index, I would like to tell you something:

1. Plagiarism can be intentional or unintentional but none of these two is allowed.

2. In original research article, most of the plagiarism is common in methodology (materials and methods) due to following the similar protocols or methods.

3. First check where (section) it is more, if it is in methodology then it may be allowed. But if it is coming in other sections of the manuscript then it is not OK and you have to minimize it.

4. It also depends on the similarity index.

e.g. similarity index of 11% from 11 source or 11% from 1 or 2 source.

5. Therefore, we must consider to minimize the single source similarities.

6. Smaller similarities from different sources is not problematic.

You also asked that is it OK for Elsevier journal or not?

See, there are a number of journal published by Elsevier. It is completely depends on journal and vary from journal to journal.

Although, 11% is not too much but try to make it less than or up to 10%.

Hopefully it will help you.

I will be happy to get your feedback that you can share (your experience) with me after getting published or communication to journal.

Best wishes

Thank you.

Thanks for your answer, it is helpful