Why is it important to learn new techniques than banking on century old DOH measure?

The Days of Supply is a thumb rule calculation used to manage inventory during the era of green accounting spreadsheets. It has many challenges including the demand fluctuations which can affect the metric. The forecast is uncertain and has an error component. So the Days of supply compounds it by basing the safety hedge on a forecast.
Some companies base it on the average historical demand, which can be even worse since the supply chain is presuming that the average demand is a better forecast than the demand plan.

Safety stock calculated in absolute numbers and driven by hard parameters is more optimal than the days of supply method. In my own consulting, I have seen companies cut down on inventories and increase service levels using algorithmic safety stock methods. At DemandPlanning.Net we preach the application of this principle combined with a good demand planning process to deliver an accurate forecast.

There are three major software providers in this area among many others. These include Optiant, Smart ops and Tools group. Ilog is another company that has a comparable tool.

These tools attempt to optimize your inventories and network. The premise is that you need the appropriate inventories to meet customer demand based on the forecast plus the forecast error. This is a short term tactical requirement. In the long-term you may want to optimize the network, by adding or deleting or replacing your inventory locations. This is an inventory placement decision but also a network optimization decision that needs to take into account other logistics costs.