Education: Negative Graduating Premiums?

I have been studying “negative graduating premiums,” a phrase I picked up in the United Kingdom. It compares the incomes of those who take on degrees versus those who learn a skill. Which fares better in the short term and long term? Do graduates earn less than their hand-skilled counterparts? If so, that is a case of a negative graduating premium. In simple terms, these can be explained as making educational investments that don’t yield commensurate returns.