The blah blah sector

Health sector is different from education sector in the way research is conducted.

Definite reference ('the')

In the phrase 'health sector', health tells us this sector, exactly.

This is called definite reference. Both writer and reader understand exactly which thing is being referred to - in this case, this sector exactly and not some other sector.

If you want to communicate this exactly then you need to use the definite article - the:

The health sector is different from the education sector in the way research is conducted.

If you do not use the when you mean this one exactly then you will receive a low score in IELTS for grammar and also perhaps for coherence and cohesion.

If you do not use the when you mean this one exactly then your reader will stop reading and think "Does he mean this one exactly, or does he mean one of many, or does he mean all of them everywhere?" You must communicate one of these meanings if you want to be understood clearly.

Indefinite reference ('a/an')

If you want to communicate one of many then you need to use the indefinite article - 'a/an':

Health is a sector that requires different research approaches.

Generic reference ('s')

If you want to communicate all of them everywhere, and your noun is countable, then you need to use the plural - 's':

Government sectors mentioned in the report include health and education.

In a separate post you will find a table illustrating article choices, together with a quick pratice activity.