country <- c('A','A','A','B','B','C','C','C','C')
year <- c(2010,2011,2015,2008,2009,2008,2009,2011,2015)
score <- c(1,2,2,1,4,1,1,3,2)

  country year score
1       A 2010     1
2       A 2011     2
3       A 2015     2
4       B 2008     1
5       B 2009     4
6       C 2008     1
7       C 2009     1
8       C 2011     3
9       C 2015     2

country year score  change
1       A 2010     1     NA
2       A 2011     2      1
3       A 2015     2      0
4       B 2008     1     NA
5       B 2009     4      3
6       C 2008     1     NA
7       C 2009     1      0
8       C 2011     3      2
9       C 2015     2  -0.33

country  avg_change
1       A         0.5
2       B           3
3       C        0.55