research process - What makes a Bachelor's thesis different from Master's and PhD theses?

All the three types of research revolve around an argument, a thesis. They of course differ in terms of student level, that is complexity.

But, what makes a bachelor's thesis different from master's and PhD theses in terms of procedures of researching given that all of them may follow the same process of research, questions or hypotheses, review of the literature, methodology, results and discussion?

Answer

The PhD thesis should be on a much higher level than the Honours/Masters thesis, offering a contribution to human knowledge that is of a sufficient level of "significance" to warrant publication in a respected journal.

Significance is highly subjective, and you also do not necessarily have to publish to be awarded the PhD (sometimes the peer-review delay means that they come out afterwards, or there may be some intellectual property issues that make it beneficial to refrain from publication). It is awarded based on your supervisors consent and a review of academics in your field. So the "significance" would probably be judged by them in terms of how much original work they see as a reasonable expectation at that stage of your development (first 3 years of serious/committed research). Unfortunately it also means that some people who probably do not deserve PhD's are awarded them anyway for fulfilling grunt work for their easy-going supervisors.

It is possible that some Honours/Masters thesis might even be more significant/higher quality than a PhD thesis. Unfortunately, this does not mean that the submission of the thesis will award the degree that they deserve. The university may have a policy to upgrade the student's enrolment if the supervisor senses that such progress is being made. However, it is impossible to upgrade to a PhD without completing Honours and I believe nearly every single university has a policy of a minimum period of enrolment before submission is allowed. A subsequent question that you may have is how to gain a PhD without enrolling in one, which is another level of achievement completely.

As for the difference between Honours/Bachelor and Masters it would depend on your university, but both have no requirement for publication quality research and are usually small tasks/ideas that are not worth the supervisors time to think about alone, or involve a lot of labor. In fact, in my school, many Honours thesis are of a higher level than the Masters, because the smart Honours students will either graduate into the work force or go straight into a PhD. The Masters students are usually those who cannot find a job and are not suited to research. However, I believe some other universities may require a mandatory Masters degree to start the PhD.

You may get a better idea by looking at some titles/abstracts of completed theses. The PhD level will be something like a new method/observation/application whereas the Masters/Honours will be an application specific set of measurements/simulations or even simply a literature review to gauge the needs of future work. The word limits are also typically different (although note that quality is NOT proportional to the number of words), with PhD at 100K, Masters at 50K and Honours at 30K at my university.