Q: Passage: Profitability index (PI), also known as profit investment ratio (PIR) and value investment ratio (VIR), is the ratio of payoff to investment of a proposed project. It is a useful tool for ranking projects because it allows you to quantify the amount of value created per unit of investment.

Question: Based on this passage, the profitability index of an investment project is the ratio of the?
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To answer the above question, the relevant sentence is: Profitability index (PI), also known as profit investment ratio (PIR) and value investment ratio (VIR), is the ratio of payoff to investment of a proposed project. So the answer is payoff to investment.