Variable cost in project scheduling software applications including RiskyProject is not defined directly. It is defined by resources (material, cost, and work) assigned to the tasks. If resource cost is prorated, task's variable cost will depend on duration of the task. Since in risk analysis task duration is uncertain (variable), cost associated with the task will be also variable. You can also define uncertainties associated with rate of resources. Some risks can affect the rate: the rate can increase or reduce if the risk occurs. It will also contribute to the variable cost of the task. RiskyProject does not have risk or uncertainties associated with units of resource allocation.