> weight_gain <- ChickWeight %>% 
+     group_by(Chick) %>% 
+     summarise(weight_gain = max(weight) - min(weight))
> weight_gain
Source: local data frame [50 x 2]

    Chick weight_gain
   (fctr)       (dbl)
1      18           4
2      16          16
3      15          27
4      13          55
5       9          58
6      20          76
7      10          83
8       8          92
9      17         100
10     19         114
..    ...         ...