Return on investment (ROI) is the ratio between net income and invested resources. When someone says that a project has a high ROI, it means that the project has an effective allocation of time, money, and other resources. In programmatic advertising, ROI is a way to gauge the efficiency of an ad campaign.
ROI is a popular indicator because of its relative simplicity and versatility. The most basic way to calculate ROI is subtracting the cost of investment from the gain of the investment, dividing the result by the cost of investment, and multiplying everything by 100x. More about the ROI calculation can be found here.
Since ROI does not take into account the factor of time, this metric can’t be used to gauge long-term investments without a modification of the original formula.