A durable commodity, expected to last for say 10 years, would have its variability V_J computed as 1/10 = 0.10/year. Maintaining a sector's stock of good J at present levels would require its continuous replenishment at a rate given by:

(0.10/year) (current stock in physical units) =

(replenishment rate in physical units per year).

Increasing a sector's stock of good J would require its acquisition at a rate above the replenishment rate; decreasing stocking levels would be achieved by setting acquisition below replenishment. The input rates E_IJ of good J to the production functions of the sectors I is computed at any given moment by the replenishment rate for the stocking level at that moment.

Delay mechanisms control the physical levels, through time, of all the commodities present in an SFEcon model. This consideration requires that the physical characteristics of each commodity J be examined so as to disclose a V_J having the same operative significance that V_J = 0.10/year has for durable goods such as capital equipment.

For much less durable goods, such as agricultural produce, these considerations would naturally center on these products’ inherent perishability. We know, for example, that the pipeline for foodstuffs stretches over about 15 days, i.e., the delay in getting the most typical farm product from the field to the dining room table. We may also observe that the prices of commodity futures tend to their extremes as variations in stocking levels tend to starve or over-stuff the accustomed pipeline delay for a given product. Such are the considerations that would dictate the V_J for an agricultural commodity.

Some further exploration of this notion follows from the impact that it might have upon a nation's chart of accounts. The US Bureau of Labor Statistics, for example, groups forestry and fishing together among the activities it places in the agricultural sector. Forests and fishing grounds are related in that rights for their harvests tend to be government concessions. A tree is like a corn stalk in that it grows out of the ground until someone cuts it down and puts it to use. Fishing is like hunting; and both have much in common with animal husbandry.

But, valid as these considerations might be per se, they have no appreciation for the economic order's temporal operations. Optimal enjoyment of a landed trout dictates its consumption at the evening meal on the day it is caught. Economic consideration might well dictate that felled tree remain in the log deck of a lumber mill for years before it is sawn into lumber. Clearly the time-factors V_J/year by which a board-foot of timber and a board-foot of flounder are transformed into an input rate (board-feet/year) cannot be the same. Thus these commodities must be counted among the goods produced by different sectors. (A few more of these considerations are apparent among the consolidations that have been made of various nations’ accounts in arriving at SFEcon's current definitions of its economic sectors.)

Another slant on V_J is apparent in the highly consolidated, macroeconomic versions of the SFEcon models: if all economic activity is to be epitomized in a single good J, then what is V_J? The answer here is probably to make a constant V_J of the capital/output ratio; i.e.: this general V_J would be computed by inverting the length of time (as a value-weighted average) that would be required to recreate everything that currently exists, given current rates of output. This would put V_J at approximately 1/3 per year.