Lately, I've been hearing people say "based off" instead of "based on," and my research shows that the use is increasing rapidly. Check out the chart.
Since I’ve started teaching, one thing that has jumped out at me is how many of my students, and also people I just hear in the halls, say “based off” instead of “based on.” As in, I believe we’re all doomed based off what I saw last night instead of I believe we’re all doomed based on what I saw last night.
I’m usually skeptical when other people report hearing some new error that annoys them because when you do research, you almost always find that people have been saying the annoying thing for a long time. I applied that same skepticism to myself, and I was surprised to find that it does look as if people saying “based off” is relatively new as far as language change goes.
There was a dramatic spike in the number of people saying “based off” instead of “based on” in the mid-1990s, at least in Google Books, which mostly includes edited text. So people were probably saying it earlier because it takes a while for nonstandard usages to make it into books and for books to be published in general. I still should have heard it before 2014, but it’s not 100 years old either, and the chart of the increase looks like some Silicon Valley startup growth chart—like a hockey stick.
Also, when you click on the older data points, they tend to be sentences in which the word based is next to off, but for some other reason. For example, a 2001 networking book talks about a “hardware-based off-the-shelf product.”
I can’t tell you why people started using the wrong phrase. English prepositions are often idiomatic, which means that there’s no logical reason that we use one instead the other. When people ask why New Yorkers say they stood on line when almost everyone else says they stood in line, the only answer I have is that it’s a regionalism, and when people ask whether they should say they are in a restaurant or at a restaurant, all I can tell them is that either is fine.
I can tell you that the correct, Standard English phrase is based on, and when you compare the Google Book Ngram results for based off and based on, you reassuringly find that based on is still vastly more popular.
Stick with saying your assumptions are based on what you saw last night, but don’t be surprised if you start hearing people use based off. It’s on the rise.