What is the best way of using abbreviations? Can I use them frequently or rarely?
For example, I use "Expectation Maximization" 4 times in first 3 sections, but I have to use it 10 times in section 5 and I have limited space. So can I start using EM (Expectation Maximization) starting from section 5? Or if I am going to use an abbreviation does it have to be in the entire thesis starting from the first usage of the abbreviated word pair?