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Ex ante or ex post: locate a French expert assessment in time

Edited by Chris Lee · Updated · About the library

Distinguish ex ante and ex post in French expert discussion. Locate forecasts, decisions and observed outcomes on a project timeline.

In short

Ex ante evaluates before the event or decision; ex post evaluates after the event using the information then available. Neither label guarantees accuracy. An early forecast and a later measured result answer different temporal questions and should not be silently substituted.

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An empty meeting room at COP22, with rows of chairs facing the stage.
Ex ante and ex post locate an assessment before or after the event being assessed. Takver · CC BY-SA 2.0 · https://creativecommons.org/licenses/by-sa/2.0/ · resized and converted to WebP

In the project below, a library tries longer opening hours. The stages distinguish assessment before implementation from assessment after the trial. An ex post evaluation may still contain uncertainty about causes.

A project and two assessment moments

  1. Before the decision
    Une estimation ex ante prévoit les coûts et annonce les hypothèses.An ex ante estimate forecasts costs and states its assumptions.
  2. At the decision
    Le comité autorise un essai et fixe les indicateurs.The committee authorises a trial and sets its indicators.
  3. During the trial
    L’équipe recueille des observations sans les confondre avec la prévision initiale.The team collects observations without confusing them with the initial forecast.
  4. After the trial
    L’évaluation ex post compare les résultats observés aux objectifs.The ex post assessment compares observed results with objectives.

Question 1No account needed
Which is ex ante?

Choose one answer.

Answer and explanation

An estimate made before implementation. The timing relative to the event defines ex ante.

Question 2No account needed
Does ex post prove causation automatically?

Choose one answer.

Answer and explanation

No. Later observation can show results without isolating what caused them.

Question 3No account needed
Can a report written later quote an ex ante forecast?

Choose one answer.

Answer and explanation

Yes. The date of the report does not change the original forecast’s status.

Question 4No account needed
What should a fair comparison preserve?

Choose one answer.

Answer and explanation

The forecast’s assumptions and the observed outcome’s scope. A meaningful comparison keeps assumptions, measures and scope visible.

Do not give a past decision information it did not have

A later outcome can show that a forecast failed. It does not by itself show that the earlier decision ignored evidence available at the time. To make that stronger criticism, identify what was knowable then and how the decision treated it.

Ex ante, le coût semblait soutenable sous ces hypothèses ; ex post, la hausse des dépenses impose de les réexaminer.

Beforehand, the cost appeared sustainable under these assumptions; afterwards, rising expenditure requires them to be reconsidered.

These Latin expressions are common in expert prose but need not replace clearer French. Avant la mise en œuvre and après l’essai often make a public account easier to follow. Keep the temporal distinction whichever wording you choose.

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