Here is some analysis on why these variables are often considered and what else is important. https://www.sciencedirect.com/science/article/abs/pii/S0263786398

Here is some analysis on why these variables are often considered and what else is important. https://www.sciencedirect.com/science/article/abs/pii/S0263786398000696 https://www.sciencedirect.com/science/article/abs/pii/S02637... https://www.researchgate.net/publication/305462896_Project_managament_cost_time_and_quality https://www.researchgate.net/publication/305462896_Project_m... More recent publications replace "quality" with "scope" since quality cannot be controlled directly and is seen as a result.