… difference between T^ (t=t) and T (t=0). You will find out a comparable population for T (say T*). You will look at the difference between T* (t=t) and T *(t=0) and declare it as the difference between T^ (t=t) and T (t=0). Finding a comparable population is an art. Advanced statistical methodologies help, but domain knowledge helps even more. Some call it synthetic control.