I have a dataframe here: each subject does 6 trials, there are 105 subjects.

I want to find the mean of 'skip' for 6 trials for each subj.

How do I start?

```
>     subj entropy n_gambles trial choice
1      0    high         2     0   skip
2      0    high         2     1   skip
3      0    high         2     2   skip
4      0    high         2     3   skip
5      0    high         2     4   skip
6      0    high         2     5   skip
7      1    high        32     0    buy
8      1    high        32     1    buy
9      1    high        32     2    buy
10     1    high        32     3    buy
11     1    high        32     4    buy
12     1    high        32     5    buy

```