Australian historian Peter Shergold confirms the findings of many scholars that the standard of living for US industrial workers was higher than in Europe.  He compares wages and the standard of living in Pittsburgh with Birmingham, England. He finds that, after taking into account the cost of living , the standard of living of unskilled workers was about the same in the two cities, while skilled workers had about twice as high a standard of living. The American advantage grew over time from 1890 to 1914, and there was a heavy steady flow of skilled workers from Britain to industrial America.  Shergold revealed that skilled Americans did earn higher wages than the British, yet unskilled workers did not, while Americans worked longer hours, with a greater chance of injury, and had fewer social services. Nationwide from 1890 to 1914 the unionized wages in manufacturing rose from $17.63 a week to $21.37, and the average work week fell from 54.4 to 48.8 hours a week. The pay for all factory workers was $11.94 and $15.84 because unions reached only the more skilled factory workers.
Question: How many total years did the American advantage grow over time?
Answer: 24