May 17, 2012

Weekly Language Usage Tips: A priori, a posteriori, ex ante, and ex post & toward or towards

Posted in a priori/a posteriori, ex ante/ex post, toward or towards at 9:39 am by dlseltzer

Tip 1: A priori, a posteriori, ex ante and ex post

A reader writes:

Here’s one if you’re interested:

When should we use “a priori” and “a posteriori” versus “ex ante” and “ex post?”

I’m not sure, but I think I have often used “a priori” incorrectly.  You’ll often see someone referring to hypotheses as a priori assumptions.  I now think “a priori” may actually refer to claims to knowledge held as self-evident truths or as presuppositions that are not subject to data (because they govern how the data is assessed or what counts as data, etc.)  But I think empirical hypotheses should rather be said to be held ex ante–that is, as inputs to the experiment, where the conclusions are outputs or ex post.

Any thoughts?

My head hurts just contemplating this, but here goes. Keep in mind that I am way out of my comfort zone, folks. My understanding is that these terms are used in philosophy, law, and economics—as I said, out of my comfort zone.

Let me say this right off the bat. I don’t think it is appropriate to use any of these terms when talking about hypotheses. I’ll explain why later.

I agree that the reader was probably using the term ‘a priori’ incorrectly; he is not alone. While many people believes that ‘a priori’ means ‘before we test it,’ it does not. ‘A priori’ refers to something that just IS—independent of evidence, independent of experience, independent of experimentation. So  ‘a priori knowledge’ is something we just know. We don’t have to test it, and we don’t plan to.

‘A posteriori,’ on the other hand, refers to something that is known based on experience or evidence. I said I would explain why these terms are not appropriate to use with respect to hypotheses. This is why. You would not have an a posteriori hypothesis since something ‘a posteriori’ is known, and hypotheses are not known for certain. You would not have an a priori hypothesis either for the same reason. ‘A priori’ refers to something that is known, and a hypothesis is not known yet—it has not been proved or supported. Does that make sense?’

‘Ex ante’ refers to something that is based on expectations or predictions of the future. So ex ante hypotheses—hypotheses that are based on your prediction of the future seems conceivable. But really, isn’t that what hypotheses are—assumptions or predictions of what will happen in the experiment. So it seems to me that an  ‘ex ante hypothesis’ would be redundant. So for ‘ex ante,’ too, the reference to hypotheses does not make sense.

Finally, and with much relief, there is ‘ex post.’ Ex post refers to the resolution of something once the uncertainty has been resolved, that is, the value is estimated on past events. Since hypotheses are tentative explanations or expectations based on observations or assumptions that can be supported or refuted through experimentation, I can’t see how ‘ex post’  relates to hypotheses in any way. I did mention I was out of my element, right?

My advice would be this: unless you are a philosopher, an officer of the court, an economist, or a business person, stay away from these words. They don’t work well as jargon in the world of medical research and science, and using them would do more to obscure the message than enhance it.

Now, before you think I am some kind of anti-Latin snob, I took many years of Latin in school. and I am very grateful to the language which helped me do well on my English SATs. But we’re aiming for clarity and precision, right? And I just don’t think this is the way to get either.

Tip 2: Toward or towards

A reader writes:

WLUT topic: toward or towards?

Yay! Back in my comfort zone.

This topic usually comes up once or twice a year.  It’s been a while now (February, 2011), so I will take another stab at it. I’m going to start with the bottom line (I’ve been doing that a lot lately—I’ll tell you why in a bit):

Use ‘toward’ in ALL of your professional writing.

Don’t EVER use ‘towards,’ or at least reserve it ONLY for your most casual writing.

The reason I gave you the bottom line first is because that is the message I want you to come away with, and I don’t want you to get hung up on this: both words are correct and can be used interchangeably.

Yeah, yeah, so officially, they are equivalent, but ALL of  the style manuals (at least the American ones) prefer ‘toward,’ and ‘toward’ is what we should stick with.

I will note that in British English, ‘towards’ is the preferred form.


So, in a fit of symmetry, I will end with the bottom line, too.

Use ‘toward,’  without any ‘s.’  The end.



  1. […] fancy sounding but often misused Latin phrases should and shouldn’t be applied to, check out the Language Use Weblog’s post on the subject). As I was saying: the point of the null and alternate hypothesis existing in the […]

  2. ergopropterhoc said,

    Apologies for resurrecting an old post, but I would have to disagree with you discussion on the use of a priori when applied to empirical research.

    Empirical research borrows the terms from statistics rather than philosophy. In this context, a priori refers to beliefs that you have formed up to the point before you run on study. Strictly speaking, it doesn’t mean “before the study,” it just means “what we believe given what we know/reasoned/felt up to this point.” For most empiricists, though, that “point” happens to be before a study, so such a distinction is generally trivial. Of importance is that the use of a priori here refers to beliefs and not knowledge. The corollary of a priori in empirical research is not a posteriori but post hoc. Post hoc refers to beliefs that you have formed based on the appearance of data.

    Note that, in empirical research, a priori and post hoc often refer to the source of these beliefs. As such a single belief does not transform from a priori to post hoc after data is collected. Rather an a priori belief can be confirmed by data (but the source of that belief still existed before you collected data). A post hoc belief wasn’t considered prior to data collection. The source of the belief arose from observing the data and not from any prior expectation. This distinction is important as beliefs held a priori and confirmed by experimentation are generally more credible than beliefs that are formed post hoc based upon data collected. Anecdotally, think of someone with an a priori belief as having made a prediction (“Based on the cloud formations, I believe it will rain”). If this prediction is confirmed (it actually rains), you can be more confident in this person’s predictive ability. Someone with a post hoc belief is simply reinterpreting the past (It actually rained *and then* the person said “I knew it was going to rain based on the cloud formations”). In the latter case, we have less of a reason to believe that this person’s beliefs are well-founded.

  3. Tom Lynch said,

    In research an assumption can be “a priori”. Example: You are researching teaching reading to first and second graders. You assume a priori that the children will have difficulty with words based on their length. Therefore you select from the reading texts the 6 longest words to pre-teach the children before they read aloud the text. You believe that this will improve their oral reading.

    After you run the study and record the words they miss as they are reading you find that length does correlate to missed words during oral reading but not always. However, now that you have data, you use it to develop the probabilities of a child stumbling on a given word. This is “a posteriori” since you have data. Using this information you can revise your pre-teaching words.

  4. […] should it be ex ante & ex post instead? […]

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