“One of the most frequent corrections we have to make to content submitted to the journal is to change ‘data is’ to ‘data are’. While there is still controversy about which is correct since both are commonly used as interchangeable, by definition 'datum' is the singular and 'data' represents the plural.”
This quote is from an editorial of the British Dental Journal, written by its Editor-in-Chief. The opinion that the word data is plural by definition is still held by many academic journals, in particular those in the medical field. However, other branches of science, notably computer science, almost exclusively use data as a singular noun. So, which is correct: This data is… or These data are…?
The short answer is: both are correct! The English word data was borrowed from the Latin language. In Latin, data is the plural form of datum, which is why some journals and style guides maintain that data can only be plural. Data, however, is now a fully functional English word and Latin grammar rules are therefore irrelevant. It is not uncommon for foreign plural nouns to be used as singular nouns in English. Another example is the word agenda, which was also a plural noun in Latin. But nobody claims agenda to be a plural noun in English! We never hear or read sentences like: The agenda for today’s meeting are as follows...
Now that we have cleared that up, we can take a look at how data is used in English today. Data can be used as a mass noun (like snow, gold, information), which cannot be counted and only has a singular form. It can also be used as a count noun (like laptop, chair, fact), which has a singular and a plural form. There is only a slight difference in meaning between them.
As a mass noun
When used as a mass noun, data is always singular. In popular writing, this is the standard usage. The singular is best used when you are describing the entire set of observations arising from an experiment. You are then referring to data as one mass: This data is reliable.
As a count noun
When data is plural, it is used as a count noun. This is a more formal and academic usage and is not found in everyday language. As a plural, data describes a large group of separate observations. When you are referring to these individual parts of data with the aim of comparing them, you can use the plural: These data are not all equally reliable.
When deciding between the two
Of course, when the journal you want to publish in prescribes the plural use in their style guide, you have no choice (Perhaps you could send them a link to this blog and see if they change their mind!). But if you do have a choice, think about what exactly you want the word data to express. You can also take a look at some papers in your own field of science to see which usage is more common and base your choice on that.
Sources:
Rosenberg, Daniel. “Data before the Fact.” In Raw Data Is an Oxymoron, edited by Lisa Gitelman.