如何为组内的每个值制作一列中位数？

Question

I have three columns: A BatchID, UnitID, and Score.

At the moment, the data set looks something like this:

BatchID      UnitID           Score
A123         A123-100         0.111
A123         A123-101         0.121
A123         A123-102         0.101
A123         A123-103         0.102
B456         B456-200         0.211
B456         B456-201         0.221
C789         C789-001         0.199
C789         C789-002         0.189
C789         C789-003         0.192
C789         C789-004         0.201
...          ...              ...

I want to add a column "median" that gets the median of the score each BATCH, and places it next to the rest of the data (repeating the same median value for each Unit in a unique Batch). Something like this:

BatchID      UnitID           Score      Median
A123         A123-100         0.111      0.1065
A123         A123-101         0.121      0.1065
A123         A123-102         0.101      0.1065
A123         A123-103         0.102      0.1065
B456         B456-200         0.211      0.2160
B456         B456-201         0.221      0.2160
C789         C789-001         0.199      0.1955
C789         C789-002         0.189      0.1955
C789         C789-003         0.192      0.1955
C789         C789-004         0.201      0.1955
...          ...              ...        ...

I tried groupby, among other things, but given that I don't really know how to use it in this case, that's not giving me the desired output.

Thank you!

Answer 1

Use groupby with transform :

df['Median'] = df.groupby('BatchID')['Score'].transform('median')

Output:

  BatchID    UnitID  Score  Median
0    A123  A123-100  0.111  0.1065
1    A123  A123-101  0.121  0.1065
2    A123  A123-102  0.101  0.1065
3    A123  A123-103  0.102  0.1065
4    B456  B456-200  0.211  0.2160
5    B456  B456-201  0.221  0.2160
6    C789  C789-001  0.199  0.1955
7    C789  C789-002  0.189  0.1955
8    C789  C789-003  0.192  0.1955
9    C789  C789-004  0.201  0.1955