I was reading the posts in one of the other threads, and I suddenly started wondering, "How do you really know if you are capable of completing a PhD.? I thoroughly believe that not everyone can do this. From the little I have seen, I would place possible Ph.D candidates in the upper 10 percent of the population. My question is, what objective academic criteria can one use for "measuring" themselves in this manner? For instance, if someone consistently scores A's on research and writing projects at the Masters level, does this indicate they are capable of the level of writing and thought necessary to complete a Ph.D? I would think this would be the best indicator of one's ability, rather than one's GPA. A GPA can be influenced by other factors, such as extra work, "open book" tests, etc., which might lead a person to overestimate their ability. What say you?