Any question answered...

Data is a singular used to describe a set volume of information. Datum is also a singular used to describe a single elemental factor of that volume of information.

i.e That datum is inconsistent with the trend of the data.

Data as a plural noun to describe multiple sets of information is pluralised the same way that sheep is.

However, ISTAT data on agricultural production do not seem very reliable for the last quarter of the nineteenth century...
This is still a singular noun by the way because it refers to one set volume of information and therefore should be does and not do.