I would like to ask you a challenge on this data frame. I do not know how to aggregate() rows in summing by combining two levels of "year factor". Find a model of my df.

```
placette year                   SP1               SP2  ... SPX
1         1 2013                 43                 4      ...
2         2 2013                 30                 0      ...
3         3 2013                 23                 3      ... 
4         1 2014                  0                 0      ...
5         2 2014                  2                 2      ...  
6         3 2014                  5                 0
7         1 2015                 16                 3
8         2 2015                 16                 1
9         3 2015                 20                 0
10        1 2016                 54                 4
11        2 2016                 51                 2
12        3 2016                 51                 0

```

I need to sum values of variables SP1, SP2, to SPX by period. For example, for 2 years (2013+2014) and three years (2013+2014+2015)...

I will expect this :

```
placette period                 SP1               SP2    ... SPX
1         1 2(2013+2014)         43                 4    ... ...
2         1 3(13+14+15)          59                 7    ... ...
3         1 4 (13+14+15+16)     113                11    ... ...
4         2 2 (13+14)            32                 2    ... ...
5         ... ... ...            ...              ...    ... ...

```

**in keeping my factor "site".**

I am working on R and I want to resolve it.

Thank you so much to help me.

Regards,
Thomas.