Modeling time periods

By default, a model is a single period model. This one period is the length of the analysis. This is often referred to as the horizon of the model.

In reality, however, not all demand is the same. It can be modeled over incremental periods of time. A "period" is considered to be some length of time in which events such as demands or inventories occur. A period could be a day, a week, a month, a year, or anything in between, and each period may be a different length.

In addition to variation in demand and inventory, costs and constraints can vary across time periods, and optimization will use period-specific values. When setting costs and constraints in multi-period models, rules apply as described in the following sections.

Tables in which Period is a required field are considered multi-period tables. These tables allow you to define period-specific values as needed.

Costs in multi-period models

If you set a cost on a regular table (standard table), that cost value is used for each period, unless the cost is also set in the corresponding multi-period table. For the period(s) for which the cost is set on the multi-period table, this cost is used and not the one set on the standard table.

Costs in periods of varying lengths

Generally, all periods in a multi-period model are of the same length. However, if your model uses months as periods or if a leap year is included in a model with yearly periods, keep in mind that an average daily cost is calculated first, then multiplied by the number of days in the period:

For leap year, the number of days is 366:

(Cost / 365) * 366

For months, the number of days varies from 28 to 31 and the number of days used for the average daily cost is 30:

(Cost / 30) * 31 for months including July and August

(Cost / 30) * 30 for months including April and June

Constraints in multi-period models

Constraints include fields such as Inbound Capacity and Outbound Capacity on the Inventory Policies table.

If constraints are set on the standard table only, the constraint is applied to period 1 only.

To apply the constraint to periods other than period 1, you must set it explicitly on the multi-period table for each period to which it should apply.

The exception to this is the Fixed Operating Cost for sites and work centers. The Capacity Period, Capacity Basis, and Apply to Horizon fields define how these work across periods. Note that Fixed Operating Cost can still be overwritten for specific periods in the multi-period tables.