@book { Porters1980GenericStrategiesPerformanceandRisk,
    title = "Porter´s (1980) Generic Strategies, Performance and Risk",
    author = "Jan {Eldring}",
    year = "2008",
    publisher = "Diplom.de",
    address = "Hamburg, Deutschland",
    doi = "10.3239/9783836623247",
    url = "https://m.diplom.de/document/226201"
}