The researchers studied how different types of businesses in U.S. cities between 1880 and 1930 affected how well workers did their jobs. They looked at data from factories. They found that when cities had more specialized businesses, productivity went up faster. Also, having a variety of businesses helped productivity in big cities, but not so much in smaller ones. This could be because businesses in big cities worked together and shared ideas to improve. Specialized industries grew a lot in cities due to better transportation. This growth led to big increases in how efficient workers were in doing their jobs.