Criminal Sentence 85: Compound Subjects

From an article about surgery:

"The number and size of the incisions depends on the surgery and equipment needed."

There are two subjects here ("number" and "size")--a compound subject--but a singular verb ("depends"). Compound subjects contain an "and," so if you are connecting two items with an "and," you need a plural verb. It's easy to forget about the first part of the compound subject, so that's why this is such a common error.

Here's the rewrite:

"The number and size of the incisions depend on the surgery and equipment needed."