Learn Swedish: Vi ser en kvalitetsmässig skillnad. - We see a quality-related difference.

The suffix -mässig is added to nouns to create adjectives meaning pertaining to, in terms of, or related to something. By adding it to kvalitet (quality), we get kvalitetsmässig, meaning quality-related or in terms of quality.

Why is there an extra -s- in the middle of kvalitetsmässig?

That is a linking -s-, sometimes called a fog-s in Swedish. When attaching a suffix like -mässig to a noun, or when compounding two nouns, an -s- is often inserted to make the word easier to pronounce, especially after words ending in a hard consonant.

Why doesn't the adjective end in -t here, like kvalitetsmässigt?

Because it describes the noun skillnad (difference), which is an en-word (en skillnad). The adjective must agree with the noun, so it stays in its basic form kvalitetsmässig. If it were describing an ett-word or being used as an adverb, it would take the -t ending.

Could you just say skillnad i kvalitet instead of kvalitetsmässig skillnad?

Yes, skillnad i kvalitet (difference in quality) means the exact same thing and is very common. Using the -mässig suffix simply elevates the language slightly, making it sound more professional, analytical, or formal in written Swedish.