The following DataFrame (df) has 3 columns : `Event`, `Counts` and `Length` where the `Counts` column contains series of numbers. I would like to estimate the mean of each `Counts` row and display it on a fourth one.

```
df = 
Event                                                 Counts Length  
0     [111240.0, 110530.0, 108540.0, 108540.0, 10838...       5                                                      
7     [117240.0, 119160.0, 120160.0, 121950.0, 12195...       6
13    [180420.0, 180420.0, 183840.0, 183840.0, 18384...       6
16    [160260.0, 159430.0, 159430.0, 157720.0, 15955...      14

```

I have tested the following expression without success.

```
 df['mean'] = df['counts'].mean(axis = 1) 

```

and other variations based on this line.

If I understand correctly, the problem due to the fact that the `Count` column contains more than one element. 

I appreciate your help for this apparently easy problem.