The Probabilities form is accessed from the Cost Data Entry form and allows you to specify a probability for a cost item. When you do this, you are implying that the cost item in question may or may not occur, and you are specifying the probability that it will occur.
You can enter a probability directly into the box at the top of the form, or you can select a probability from the list given in the lower half of the form. This list actually specifies a probability range, such as 40% to 60%, and the value you select will be a mid-point value in that range. The number of ranges in this list will be the same as the number of impact scale levels you have elected to use, which will in turn depend on your risk matrix size.
There are two types of probability ranges you can use, equal intervals and decade intervals. Use decade intervals for low probability events such as earthquakes, fires and floods, and equal intervals for typical management-related events such as a supplier delivering late.