Aggregation acknowledges that the two classes have their own life cycle (i.e. can exist without each other). Each object is instantiated separately. In the example of a hand of cards, the cards will already exist before they are added to a Hand class.
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By aggregating all the data points, we can identify the source of traffic and see how they progress through the customer journey . In turn, that helps us understand the actual value of the marketing activity.
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