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It depends on what you're dealing with. Sometimes, gathering the whole population is not possible.
Scenario1: if you have to collect information about US company profit margins in2013, it's not possible to collect such information about every company as many are privately held and don't share their financial statements. So in that case, you would collect a representative sample.
Scenario2: On the other hand, if you have to collect information about all currently enrolled students in a particular school, then such information would be available with the school authorities. In this case, yes, you would collect a sample if you were short on time.