"qualitative difference" refers to a noticeable and significant difference in the quality or nature of something. It means that there is a distinct and valuable change, rather than just a minor or quantitative variation. Full definition
The new nerves also show qualitative differences from their older counterparts. (scientificamerican.com)
I say, no, it's a huge qualitative difference between what this study shows and the statement that we think the probability that the vaccine works is 96 %. (sciencemag.org)
If they have the same qualifications and experience, (common when you have a range of people who've all met the job criteria) interview questions have to find qualitative differences. (cvtips.com)