Era
20th century
63
1945-
40
1990
23
19th century
18
1950-
11
20th century --
10
1945
8
1976-
8
United States
8
1900-
7
1949-
7
1933-1945
6
1945 -
6
1990-
6
Cases
6
Study guides
6
1918-1933
5
1949
5
1981-
5
Australia
5
Case studies
5
1871-1918
4
1914-1918 - Causes
4
1917
4
1925-
4
1939-1945
4
1966-
4
20th century. --
4
1893-
3
1917-1936
3
1949-1976
3
1960-
3
1961-1975
3
1975-
3
1979-
3
1981
3
1981-1993
3
1999
3
20th century - Juvenile literature
3
Biography
3
Study and teaching
3
stories, plots, etc
3
-Problems
2
1642-1727. Principia - Congresses
2
16th century --
2
17th century
2
18th century
2
1911-1949
2
1917-1921
2
1920-1929
2
more ...