- Manifeste pour la logistique quantitative
- Le test de la performance logistique
- Synthèse la logistique quantitative
- Prévisions probabilistes généralisées
- Optimisation fondée sur des décisions
- Moteurs économiques
- Préparation des données
- Ingénieur données logistiques
- Calendrier d'un projet classique
- Livrables projet
- Évaluer les résultats
- Antimodèles logistiques

- Prévision avancée du stock
- Rapport des priorités de commande
- Ancien format des fichiers d'entrée des prévisions
- Ancien format des fichiers de sortie des prévisions
- Choisir les taux de service
- Gérer vos paramètres de stock
- Ancien rapport Excel de prévision
- Utiliser les Tags pour améliorer la précision
- Les bizarreries des prévisions classiques
- Les bizarreries des prévisions quantiles
- Gérer le biais des ruptures de stock sur les prévisions
- Agrégations au jour, à la semaine et au mois

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Mathematical distributions are powerful and useful for modelling many business situations, especially those where uncertainty exists. Envision treats

The

Also, Envision distributions (referred as

Within Envision, distributions are materialized through a special data type named

d := dirac(42)

Distributions can be exported into a file using the Ionic data files. However, distributions cannot be exported

Envision offers many more ways to generate distributions. They will be reviewed in the following sections.

This plot has been generated in Envision with the single-liner detailed below:

show histogram "My first distribution!" a1d4 tomato with poisson(21)The

`histogram`

tile expects a single scalar distribution to be provided after the `with`

keyword.$$f+g: k \to f(k) + g(k)$$ From Envision's perspective, assuming that both

`X`

and `Y`

are distribution vectors, the same operation is similarly written as:
Z = X + YIt must be noted that even when dealing with distributions, Envision remains a

Z := X + YIn this and the following sections, whenever we use

`X`

and `Y`

in script examples, we assume that these two variables are actual distributions.Then, the point-wise multiplication and subtraction are defined with: $$f \times g: k \to f(k) \times g(k)$$ $$f-g: k \to f(k)-g(k)$$ which translates quite transparently into the following Envision syntax:

Z = X * Y Z = Z - YFrom the perspective that a number $\alpha$ can be implicitly assimilated to a constant function $f_{\alpha}: k \to \alpha$, Envision allows to combine numbers and distributions - but only if the resulting distribution is compact.

Z = 2 * X // OK, it's compact Z = X / 2 // not dividing by zero is OK Z = X + 1 // incorrect, not a compact distribution Z = X / Y // incorrect, Y is compact hence has zero valuesThe distributions can also be shifted. The shift operator is typically written as:

$$f_{n}: k \to f(k+n)$$ The corresponding Envision syntax is:

Z = X << n // left shift Z = X >> n // right shiftNaturally, if

`n`

is negative, then the shift operators keep working, but the left shift becomes a right shift, and `distrib()`

function makes it possible to regenerate this grid (by Demand = distrib(Id, Grid.Probability, Grid.Min, Grid.Max)The resulting

`Demand`

variable is a distribution. When the original grid includes segments that are longer than 1, `distrib()`

uniformly spreads the mass across the segment. The mass of the distribution is preserved by `distrib()`

.`ranvar()`

aggregator. Its syntax is:
X = ranvar(Orders.Quantity)The

`ranvar()`

aggregator returns a `ranvar()`

returns `dirac(0)`

.`ranvar.segment()`

aggregator generates a distribution from a time-series. Its syntax is:
D = ranvar.segment( start: Items.Start // first date for a each item end: Items.End // last date (inclusive) for each item horizon: Items.Horizon // length of period for each item date: Orders.Date // the date of each event quantity: Orders.Quantity) // the quantity of each eventIt computes, for each item, the distribution of the sum of event quantities for periods of horizon length entirely between the first and last date for that item. Typically, the horizon length would be the leadtime of an item.

`extend.distrib()`

function which precisely does this. The syntax is illustrated as follows:
X = poisson(1) table Grid = extend.distrib(X) show table "My Grid" with Id, Grid.Min, Grid.Max, Grid.Probabilitywhere

`X`

is the distribution vector generated on line 1 as a Poisson distribution. On line 2, the distributions are inflated into a table named `Grid`

. This table has an affinity `(Id, *)`

, and as illustrated on line 3, the table is auto-populated with the numeric columns `Grid.Min`

, `Grid.Max`

and `Grid.Probability`

. Both `Grid.Min`

and `Grid.Max`

are inclusive boundaries.When extending relatively compact distributions, the resulting table typically contains lines of +1 increments - aka

`Grid.Min`

and `Grid.Max`

increased by +1 from one line to the next. However, if we were to consider the extension of high valued distributions, for example `dirac(1000000)`

, then it would be extremely inefficient to generate millions of lines. Thus, the function `extend.distrib()`

will aggregate large distributions into thicker buckets. This explains why we have both `Grid.Min`

and `Grid.Max`

which represent the inclusive boundaries of the bucket.In order to gain more control on the granularity of the buckets generated, the function

`extend.distrib()`

offers the first overload:
table Grid = extend.distrib(X, S)where

`S`

is a number vector. The resulting table provides buckets aligned with the segments [0;0] [1;S] [S+1; S+M] [S+M+1;S+2*M] ... where `M`

is the default bucket size - also called the Finally, the second overload of

`extend.distrib()`

provides even more control with:
table Grid = extend.distrib(X, S, M)where

`M`

is a mandatory bucket size. If `M`

is zero, then the extension reverts the default bucket size, auto-adjusted by Envision. This second overload is particularly useful when Beware that

`extend.distrib(X, S, M)`

may fail depending on the capacity allocated to your Lokad account if you try to extend a high valued distribution while forcing a low multiplier.`*`

, namely:
Z = X +* Y // additive convolution Z = X -* Y // substractive convolution, same as X +* reflect(Y) Z = X ** Y // multiplicative convolution Z = X ^* Y // convolution powerThe additive (resp. the substractive) convolution can be interpreted as the sum (resp. the difference) of the two independent random variables $X+Y$ (resp. $X-Y$). The multiplicative convolution, also known as the Dirichlet convolution, can be interpreted as the product of two independent random variables.

The convolution power is more complex and represents: $$X ^ Y = \sum_{k=0}^{\infty} X^k \mathbf{P}[Y=k] \text{ where } X^k = X + \dots + X \text{ ($k$ times)}$$ This last operation is of interest because of its relationship to the process leading to an integrated demand forecast, where $X$ represents the daily demand - assumed stationary - and where $Y$ represents the probabilistic lead times.

See also our page on convolution power.