Dérivée troisième

L'inflation belge accélère à 2,4% en novembre.

Titre d'un article de l'Echo aujourd'hui.

Dérivée troisième, Ah ! Que J’aime entendre ta voix !

L’inflation est le changement de prix, une accélération est la dérivée seconde de la position d’inflation. Nous sommes donc en présence d’un article sur la dérivée troisième des prix comme fonction du temps.

Malheureusement la première ligne de l’article déçoit immédiatement les fans de la dérivée troisième, car elle indique:

L'inflation [...] est repartie à la hausse.

Seulement une dérivée seconde!

Ce que l’article n’indique pas est que l’inflation généralement commentée est l’inflation annuelle. Quand on compare l'inflation d'"octobre 2025" à l'inflation de "novembre 2025", il y a 11 mois en commun. Par conséquent, on compare novembre 2025 — qui entre dans la période annuelle — à novembre 2024 — qui en sort.

Pour comprendre ce qui ce passe, il faut aller à la source (Stabel) mais malheureusement les journaux ne citent pas leur source convenablement (juste le nom) et il faut une recherche pour trouver les chiffres bruts. Voici ma source: https://statbel.fgov.be/fr/themes/prix-la-consommation/indice-des-prix-la-consommation.

Le changement de prix en novembre 2025 a été de 0,40% de plus qu'en novembre 2024. L'inflation mensuelle octobre 2024—novembre 2024 était de 0.1657% et celle de octobre 2025-novembre 2025 était de 0,5611%, ce qui donne 0,3955% de différence (0,40% après arrondis). L'inflation mensuelle est elle-même très volatile, sur les deux dernières années, elle a varié de -0.82% à +1.38%; en comparant les mêmes mois de l'année d’une année à l’autre, on voit des changements de -0.62% à +0.90%.

De beaux articles à prévoir chaque mois!

Pour moi, un chiffre plus intéressant est l'inflation sur une plus longue période, par exemple 4 ans — le temps d'une législature. Si on utilise les chiffres depuis 2015, cela donne des inflations "législaturisées" — par opposition à annualisées — entre 5,29% et 23,11% (dernière période: 17,79%). Et cela, ça fait peur...

Malheureusement les articles de presse n'autorisent plus des commentaires sous les articles, un peu comme les "community notes" de X.

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