How common is the confusion between "affect" and "effect"?

The [mis]usage is quite common, but the two have not started to merge and should not be considered equivalent. Most lists of common vocabulary mistakes list affect vs. effect among the most frequently encountered.

Don't assume that what you see on a U.S. Representative's web page is grammatically correct or even good English. Even well-intentioned, literate people make what I call "spell-check" errors. That is, a spell-checker will validate both effect and affect and even grammar checkers may not flag their misuse since they can both appear as nouns or as transitive verbs. (It didn't get flagged by the computer? Get it on the web page!)

Proofreading is something almost no one does by hand (or eye) anymore. Too bad.

A short answer is it's pretty darn common. In fact, it falls under the most common spelling errors: Read more: Affect and Effect: Commonly Confused Words.