year   perhouse
1850     5.55
1860     5.28
1870     5.09
1880     5.04
1890     4.93
1900     4.76
1910     4.54
1920     4.34
1930     4.11
1940     3.67
1950     3.37
1960     3.35
1970     3.14
1980     2.76
1990     2.63
2000     2.59