This was in the Olden Days, when the Romans were top nation on account of their classical education…
— 1066 and All That
My curious mix of state and private education left me, to paraphrase Ben Jonson, with almost zero Latin and less Greek. Perhaps because of this, although I can be something of a language pedant, I would normally refer to, say, ‘the next item on the agenda’, and would only talk about ‘the next agendum’ in a spirit of playful fun. ‘Agendas’, however, would make me wince. I do know, deep down, that ‘agenda’ is a plural.
Similarly, even though I work with rather a large amount of data, I have almost never used the word ‘datum’ except in one of its specialist meanings. Indeed, for most science, if you have little enough data to think of each item individually, then you probably don’t have enough to draw useful conclusions!
And so, like most people, I think of data rather as a fluid substance, to be measured by the bucket-load — megabytes of data, like pints of milk or glasses of wine — rather than as a plural. Few people would say ‘there are a dozen data in this table’ unless they were classicists, who would never actually have to deal with it. So you’ll note that in the previous paragraph I said “if you have little enough data”. If I had instead opted for “if you have few enough data”, you would probably have assumed that I was making a point, one that wasn’t actually to do with the data at all.
Nonetheless, etymology has always intrigued me, and I love the fact that Norman Gray can write a long, sensible and interesting discussion of this very topic, which I can read over breakfast before going off to analyse a datum or two…