Generally,the longer the time fence the less specific the forecast should be,as resource decisions made based on long-term forecasts are not product-specific. For example, plant size is more product-flexible than plant equipment; plant equipment and labor are more flexible than purchased material. Managers can make the accuracy of the forecast less important by setting time fences and keeping the forecast (and the resources) as general as possible for as long as possible. The time fences serve as ``trigger'' mechanisms to ensure that the time period does not pass before the decision is made.