As the writer above points out, forming plurals correctly (according to the rules of the languages from which words were borrowed) is indeed a marker of education, expertise, and social class. In this sense, it’s like dressing appropriately: it has no relationship to your actual skill as an analyst, but it will affect how you are seen & treated. So, for those of you whose Latin and Greek may be a shade rusty, let’s review some plural forms. There are several types of words that have been imported from Latin and Greek into English and are still pluralized according to Latin/Greek rules.

• Criterion/a: people often use the singular and plural kind of randomly, making one or the other do the work of both.
• Datum/a: people generally don’t realize that “data” literally just means “facts” (or more literally “things that have been given”). In technical writing, it’s important to treat data as a plural—“these data”—but in speech people almost invariably treat it as if it were singular—“this data”. (Within the next 10 years, I predict that even in technical writing “this data” will be standard. For now, either treat data as plural or find a different word.)
• Axis/axes and matrix/matrices: since English words are generally pluralized by adding an s, it’s logical to assume that if you encounter a plural (axes, pronounced axeez) that ends in s, you can get its singular equivalent by removing the s (axe, pronounced axee). It is just another of the many quirks of the English language that in fact the singular still has an s on the end and is pronounced quite differently from the plural (axis, pronounced axiss).