The study looked at data from 1970 and 1987 to understand how different industries develop in cities. They found that new high-tech industries do well in big, diverse cities, but as they mature, they move to smaller, specialized cities. Older industries benefit from having a lot of the same type of industry in one place, while newer industries benefit from having a variety of industries around. This shows that cities play a big role in how industries grow and change over time.