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Uncertain variables propagate uncertainty samples during dynamic simulation. If an uncertain variable is used in a dynamic simulation, its uncertainty sample is calculated only once, in the initial time period.

'''Example:''' The following definitions model population changes over time:

 Variable Population := Normal(30, 2)
 Variable Birthrate := Normal(1.2, .3)
 Time := 1 .. 10
 Variable Pop_by_year := Dynamic(Population, Self[Time - 1] + Birthrate)

[[File:Chapter17_14.png]]

The uncertainty samples for <code>Population</code> and <code>Birthrate</code> are each calculated once, at the initial time period. The same samples are then used for each subsequent time period.

==Resampling==
If you want to create a new uncertainty sample for each time period (that is, resample for each time period), place the distribution in the last parameter of the '''[[Dynamic]]()''' function. For example, replace <code>Birthrate</code> with its definition in <code>Pop_by_year</code>:

 Pop_by_year := Dynamic(Population, Self[Time - 1] + Normal(1.2, .3))

An alternative way to create a new uncertainty sample for each time period is to make <code>Birthrate</code> a dynamic variable:

 Birthrate := Dynamic(Normal(1.2, .3), Normal(1.2, .3))
 Pop_by_year := Dynamic(Population, Self[Time - 1] + Birthrate)

==See Also==
* [[Dynamic]]()


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