Every time I run a simulation with the same parameters in the Run window I get exactly the same results. The results are different if a different number of replications is set each time the run is started
These are my settings in the run window: enter image description here
I have a lot of Process blocks. Most of them have a normal distribution in duration. Why are the results not different?
If it helps in any way, her is a photo of the constructed model: enter image description here
Arena uses the same random nr stream for your run unless you tell it to use another - so your first rep will look the same every time. The answer depends on the distributions you sample from. If you change the logic and sample at other times, the answer will change. Each following rep will have a unique answer based on the random nr streams you are sampling from. It makes it easier/possible to find errors if you can execute exactly the same run.
Arena provides (by default) ten different "streams" of (pseudo) random numbers. If you don't ask the system to use a particular stream, it will use stream 10. For example NORM(10,2) will use stream 10 to calculate a random normally distributed number (mean 10, standard deviation 2); NORM(10,2,4) will use stream 4 to calculate a similarly distributed number.
By default, the 10 random number generators for the streams are initialised at the beginning of each run to 14561, 25971, 31131, 22553, 12121, 32323, 19991, 18765, 14327, and 32535 (from Arena help). At the end of one replication, the generators will not be re-initialised, so they will start the next replication with a new value.
You can control the random number generator initialisation with the SEEDS element.
As @Marlize says, this helps to ensure that you can reproduce a simulation result if you need to.
