Complex sentence: According to the 1990 U.S. census, 948,558 people claimed be of Austrian descent (only 0.4 percent of the total population), when in the 19th century, a total of 4.2 million Austrians had immigrated to the United States.
Split into simpler sentences: According to the 1990 U.S. census, 948,558 people claimed be of Austrian descent (only 0.4 percent of the total population). In the 19th century, a total of 4.2 million Austrians had immigrated to the United States.