Answer:
You must assume that all events related are gonna follow the rate.
For example, if a tank is being filled 100 gallons per minute, you must assume that the rate is always the same, and that all variables are following this rate, otherwise you wouldn't have a good prediction.
Now, in real world, rates are only approximations, because there's always another variable which is gonna make some changes in the rate of change.
So, when you use a rate for predicitions, you assume that all other variables doesn't affect events that much to miss to hard the predicition as such.
