In 1880, she was the sole woman in her Allegheny College graduating class. She then moved on to other topics, including animal rights and women's suffrage. Tarbell took notes during her interviews with oil industry leaders, later published as two volumes titled The History of the Standard Oil Company. From these notes, she created a narrative that explained how Standard Oil became so powerful - by taking advantage of its customers. For example, it would sell oil at a low price in states where there were no gas laws, like Pennsylvania. Then, when it moved into states that had stricter regulations, it would cut prices to make up for its previous abuse. Tarbell also revealed how Standard Oil manipulated the politics and law enforcement agencies of Ohio, Kentucky, and Indiana to protect its business interests. Finally, she wrote about the many deaths that resulted from the oil industry's practices, including workers who were killed while drilling holes in roads to test the quality of asphalt. Tarbell's work prompted Congress to pass the Sherman Antitrust Act in 1890. This law served as a guide for all future antitrust cases by requiring companies to be competitive in more than one state or region of the country if they wanted to keep their deals secret.