Data is a Latin plural with a singular datum though datum is seldom used on its own any more. The Macquarie Dictionary says: ‘The connection between data the plural and datum the singular has been almost completely broken, so that while datum survives in such compounds as datum point, it does not have the frequency of use that data has.
But should data be singular or plural?
Many writers, knowing its Latin origins, insist that data must take a plural verb.
The data were analysed after they were collected.
The data was analysed after it was collected.
Both usages exist today, but often the distinction will be made based on whether you are treating data as a count or noncount noun (also known as a mass noun).
When data is a count noun (items that can be counted), the plural makes sense.
The data used were out of date.
In that type of sentence, you could replace data with another count noun such as ‘facts’.
However, when data is treated as a noncount noun (items cannot be counted), the singular makes sense.
The data used was out of date.
With that type of sentence, you could replace data with a word such as ‘information’.
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