# Probabilistic Transitions

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• #1596

I have inherited a model that has a probabilistic transition in a number of states with a probability of 1. There are also other probabilistic transitions in these states (with p<1). Is it true in this case that the deterministic transition would never happen because the p=1.0 would always be >= the random number drawn by the VDDT program algorithm?

Also, after reading the VDDT manual (excerpt below), I’m unclear what happens to a cell if there are two or more probabilistic transition types that have the same probability. Which one will be chosen if this probability value is aligned with the random number?

From VDDT manual Appx B:
“For each cell, the model makes a random draw from a uniform distribution between zero and one. If that random draw is less than the total of all transition probabilities for the cell’s current class, then a transition occurs for that cell. The decision about which transition occurs is based on the same random draw and on the relative probability of all the possible transitions. For example: Class A has three possible probabilistic transitions in this order: Fire (p=0.01), Wind (p=0.001) and Insects (p=0.02). A random number R is drawn. R≤ 0.01 would result in a fire, 0.01

#1765

In this situation a probabilistic transition will occur to a cell every time-step. Whether a deterministic transition occurs will depend on what the probabilistic transitions that occur do to the state and/or age of the simulation cell. If the probabilistic transitions move the cell to another state then deterministic transitions from a state that has Probabilities > 1 will not occur. If the probabilistic transitions set the age back so that the cell can’t age to reach it’s end age then deterministic transitions will not occur.

If probabilistic transitions occur but do not set the age back then deterministic transitions can still occur. The cell will continue to age and eventually reach it’s end age and undergo a deterministic transition.

#1766

One last comment in addition to my previous post. VDDT has an internal flag in the database that allows the user to turn aging off after a transition occurs. By default this option is turned off, however if this were turned on and a probabilistic transition occurred every time step then aging would not occur and neither would probabilistic transitions from this state.

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