Data

Data is plural; datum is singular. When dealing with statisticians or scientists, always number data and datum correctly. Outside scientific writing, however, data has found frequent use as a mass noun, sometimes with the singular data point, which has even formed the redundant plural data points.

Though I have yet to find a foolproof argument, I wonder whether the mass noun sense of data should be considered correct in any situation. Whether we like it or not, common usage will, most likely, revise the traditional plural into a contemporary singular. But, for now, prefer the traditional data are to data is.