A dissertation methodology is often confused with scientific method, especially in science fields where research is common.
However, there's a distinct difference between scientific methods and dissertation methodologies.
dissertation methodology is a distinct chapter that describes the methods by which the researcher approaches a problem and collects data through research.
The purpose of the report methodology is to define a problem, outline procedures that the researcher plans to use in order to learn more about the problem, and identify possible variables that the researcher may face during his/her research.
In other words, it should demonstrate that you took various variables into account and that you can be reasonably assured that the results are accurate.
As with any type of writing, your methodology chapter should include an introductory paragraph that describes the problem that you'll be addressing through your methodology.
Therefore, the report methodology shouldn't only help to establish that the researcher has a well-developed plan of approach for tackling his/her research, but also that the researcher is aware of inherent problems within his/her own methodology that could impact the final results.
As such, the report methodology is the first step towards establishing the credibility and authority of the researcher.
The methodology chapter of your dissertation or thesis isn't necessarily meant to provide so much detail that the reader can completely recreate the process that you used to conduct your research.
At the same time, it should be thorough enough that the reader can plainly see that you were thorough in your methods and that the methodology you utilized was sound.