Finding a threshold between two accumulations My question:
I have a list of numbers. This numbers are part of two accumulations, for each accumulation there is some unknown number of values around a specific average I don't know.
How can I find a threshold between those two accumulations, so I can say for every number if it's in accumulation $1$ or $2$?
Calculating the average of the two values forming the biggest jump would not work, it would be too unprecise. 
Almost no numbers are the same, so it's originally not a bimodal distribution.
A computer should finally calculate this, so the way of doing this can be long.
The data is made by a human, pressing a button longly or shortly. The computer should detect if he means long or short, independently of the absolute length of the pressure. 
Thanks for your advice.

I already have an idea:
Maybe I could "group" the numbers reducing their "resolution" and then calculate the threshold of the now bimodal distribution. But this "resolution" has to be right, if it's to small, the result would be too unprecise, if it's too high, the result could be totally wrong.
I'm interested in your ideas :)
