How to differentiate two subgroups from a histogram? I have a set of samples in which I assume there are 2 definite subsets in it. I plotted their values in a histogram and found that there are two distinct modes as shown in the figure below.
My question is how do I differentiate two groups. i.e how do I choose a value that differentiates the two subsets? 

 A: I assume you are talking about Neonatal Behavioral Assessment Scale values in Hereditary Renal Adysplasia. 
I often see in medical research that physicians want to have cut-offs and simple threshold based interpretations of their research results, based merely on the distribution of the measurements. Practice and applications however usually need high positive predictive value or high negative predictive value, so the characteristics of the future population tested have to be considered. My point of view is even if now you just want to "differentiate two groups" you probably want to apply this somehow in the future and thus you probably want to find the optimal threshold, optimising costs, risks and benefits (survival, quality of life etc.) in a practical setting. So I suggest that you to think these over in your application.
A: If you are willing to assume the populations have the same variance you could use essentially LDA without the normality assumption (a.k.a. Fisher's Method or Fisher's Discriminant Function).
Without this assumption you could try an EM algorithm which is indirectly what Matt Suggested since this would be a mixture model approach. 
