Permissible gap between sub-topics of a PhD thesis? I am a Ph.D. student who decided to change the direction midway. At a high level, I work on combinatorial optimization under uncertainty. In my proposal, I had said that I would use a technique A and its variants to tackle my problem. But now I have become uninterested in this direction.


I want to broaden my combinatorial optimization problem to P' so that I can consider a technique B which is totally unrelated to A, and since I broadened the problem I cannot directly compare the results from A.
Furthermore, I want to make some more changes in the problem to P'' and then use a technique C.


Now I have problems P,P', P'' which are variants of same combinatorial optimization problem under uncertainty but the techniques A, B and C are completely different and the results cannot be compared directly. Though I will compare A, B and C with techniques from literature separately. The underlying link is that all these problems are characterized by uncertain information.

Can I rope this into a thesis? Please note that I have finished my work with P, A and am starting to think about P', B. My concern is that there is a gap between these sub-topics and my Ph.D. thesis might lack cohesion. Suggestions/advice requested.
Ask your advisor. In most places, they de facto alone decide whether or not a thesis satisfies the program' requirements.
Even if, in your place, they don't, the can give your valuable information as it is their job to advise you.