> diet_mass <- ChickWeight %>% 
      group_by(Diet, Chick) %>% 
      summarise(gain_masse = last(weight) - first(weight)) %>% 
      group_by(Diet) %>% 
      summarise(gain_moyen = mean(gain_masse))
> diet_mass
Source: local data frame [4 x 2]

    Diet gain_moyen
  (fctr)      (dbl)
1      1      114.9
2      2      174.0
3      3      229.5
4      4      188.3