The first time you mention a percentage in IELTS Task 1 writing, make sure that you include both the subgroup and the supergroup:
If the Task 1 question is a pie chart, the supergroup will be given as a label above or below the chart. The pie chart at the top of the page is labelled British people, and so British people are the supergroup.
If there is more than one supergroup, just make sure you mention the name of the next supergroup when you introduce it:
If you want to avoid repetition, you can do this:
If there are 3 or more supergroups, it can be convenient to put numbers in brackets:
Notice that all of these examples include both the subgroup and the supergroup(s)!
If you don't mention the supergroup at least once in your writing, then the reader does not know which group the percentages refer to, and you will receive low scores for Task Achievement and for Coherence and Cohesion.