Era
20th century
76
1945-
45
1990
27
19th century
18
1950-
13
20th century --
11
United States
9
1945
8
1976-
8
1900-
7
1949-
7
Cases
7
1933-1945
6
1945 -
6
1990-
6
Case studies
6
Study guides
6
1871-1918
5
1918-1933
5
1925-
5
1939-1945
5
1949
5
1981-
5
Australia
5
1914-1918 - Causes
4
1917
4
1966-
4
1985-1991
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
3
20th century - Juvenile literature
3
Biography
3
Congresses
3
Malaysia
3
Problems, exercises, etc
3
Study and teaching
3
stories, plots, etc
3
-Problems
2
1642-1727. Principia - Congresses
2
16th century --
2
more ...