Frequently Asked Questions /

PRISM Properties

Why is my expected reward infinite?

This mostly commonly occurs when you are computing the expected reward that is accumulated up until some target set of states is reached ("reachability reward" properties). For example:

R=? [ F "end" ]

As mentioned earlier, this kind of property returns infinity if "end" is not eventually reached with probability 1. This is a choice that we made when designing the property specification language. Often, it is reasonable to assume that, if a path continues indefinitely without reaching a goal state, then reward will continue to be accumulated infinitely often (this would usually be true when modelling time as a reward structure, for instance). If there is a non-zero probability of not reaching the target (i.e. the probability of reaching it is less than 1), we would then expect the overall expected reward to be infinite.

You can check whether the probability of reaching the target is 1 with a property like:

P=? [ F "end" ]

A similar situation arises with models that contain nondeterminism, such as MDPs. The maximum expected reward to reach a target is finite if and only if the minimum probability of reaching the target is 1. Conversely, the minimum expected reward is finite if and only if the maximum probability is 1.

How do I check if a property is true in multiple (or all) states?

Consider a typical boolean-valued PRISM property, such as:

P<0.01 [ F "error" ]

i.e. "the probability of reaching a state labelled with "error" is less than 0.01. By default, when model checking this query, PRISM will report the result of this property for the initial state of the model, i.e. whether, starting from the initial state, the probability of reaching "error" is below 0.01. (This is in contrast to older versions of PRISM, which used to report whether the property was true for all states.)

To check whether the above property is true for, say, all (reachable) states satisfying the label "safe", you should use filters, as illustrated below:

filter(forall, P<0.01 [ F "error" ], "safe")

If you want to check whether the property is true for all reachable states, you can use either of the following two (equivalent) properties:

filter(forall, P<0.01 [ F "error" ], true)
filter(forall, P<0.01 [ F "error" ])

In older versions of PRISM, checking that a property was true in a particular set of states was done using implication (=>). If you wish, you can still use a similar form of property to achieve this, as shown by the following example:

filter(forall, "safe" => P<0.01 [ F "error" ])

How do I compute the probability of an action occurring?

PRISM's property specification language is primarily state-based, e.g. you can compute the probability of reaching a state that satisfies the label "error":

P=? [ F "error" ]

So how do you compute the probability of a some action b occurring? You need to make a small change to your model. The cleanest way to do this is to add a small module that changes state when the action occurs, e.g.:

module checker

    q : [0..1] init 0;

    [b] q=0 -> (q'=1);
    [b] q=1 -> (q'=1);


You can determine the probability of action b occurring in the model with the property:

P=? [ F q=1 ]

By design, the module above will not affect the behaviour (timing, probability, etc.) of your model at all, so all other properties will remain unchanged. This is true for any of the model types that PRISM supports. It may, though, lead to a (hopefully small) increase in total model size.

You can also modify the property above to compute, for example, the probability of b occurring within T time-units or the expected time until b occurs:

P=? [ F<=T q=1 ]
R{"time"}=? [ F q=1 ]

(where a constant T or reward structure time have been added to the model, as appropriate).

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