Abbreviation usage in PhD Thesis 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?
It's perfectly fine to define an abbreviation the first or second time you use a word and then continue to use it. You don't have to continually use the abbreviation after that, but there are not many reasons to switch back to using the full version.

Using your example:


  We examine the Expectation Maximization (EM) of underwater basket weaving techniques in tropical locations. We found that the EM is normally ...