> On what basis?

Probably the same basis by which California is the poorest: statewide cost-of-living adjusted poverty line.

There's a number of ways this measure is potebtially misleading; for instance, both New York and California are diverse states where the local cost of living varies considerably, and the numbers for any CoL adjusted measure would look very different if instead of a statewide CoL you used, say, county CoL based on residence and then aggregated the statewide numbers.

But using this or other statewide CoL-based metrics does get at something that mean or median income or measuring against the even less relevant national poverty line does not.