# Checking for plagiarism with a proportion test

I'm reviewing research proposals. The proposal is, in human eyes, practically the same as a 2017 degree thesis. The thesis guide professor is the principal investigator of the current project. I used an online site (https://copyleaks.com/) to verify the amount of similar text and it indicates 18%. I would like to add as many evidence to my decision not only to reject but also raise an expression of concern for plagiarism.

My question is: would it be correct to apply a proportion test to indicate that the proportion found, of 18% of equal text in an 18000-word document, is scarcely attributable to chance?

I used an online calculator for the calculation, available in https://www.medcalc.org/calc/test_one_proportion.php with these data

• Why did you choose $p=0.05$ as null hypothesis value? That seems artificial. I think this is a case where you need to compare with an "empirical null", you need to find what is the typical percentage of similar text between such documents in a class of documents relevant for comparison. Say, if this is a lab field, it might be natural to copy some sections such as "methods and materials" and maybe others. Jun 11, 2019 at 14:30
• Thank you David, it seems that between 10 to 20% would be acceptable, with most opinions towards a maximum of 10%. See researchgate.net/post/… Jun 11, 2019 at 14:31
• @sergiouribe If 20% is still considered acceptable, then 18% is too. No statistics needed. But the main problem I have is the reliability of that index (after all, it's summarizing into a single number some phenomenon that is way more complex than that) Jun 11, 2019 at 14:33
• Indeed @kjetilbhalvorsen . It's arbitrary and that's part of my question. I could consider that it is natural for a lab to continue with a line of research and to base projects based on previous work, but in the project the previous thesis is not cited. The project is essentially the thesis but with a major n. Jun 11, 2019 at 14:34
• @sergiouribe: * the previous thesis is not cited. The project is essentially the thesis but with a major n.* (something was cut off there) This is the main argument then, and not the hypothesis test. For that you would need to gather data as I indicated. Jun 11, 2019 at 15:49