I have a rather simple request and have not found a suitable solution online. I have a DF that looks like this below and I need to find the cumulative deviation as shown in a new column to the DF. My DF looks like this:

```
year month Curr Yr LT Avg
0 2022 1 667590.5985 594474.2003
1 2022 2 701655.5967 585753.1173
2 2022 3 667260.5368 575550.6112
3 2022 4 795338.8914 562312.5309
4 2022 5 516510.1103 501330.4306
5 2022 6 465717.9192 418087.1358
6 2022 7 366100.4456 344854.2453
7 2022 8 355089.157 351539.9371
8 2022 9 468479.4396 496831.2979
9 2022 10 569234.4156 570767.1723
10 2022 11 719505.8569 594368.6991
11 2022 12 670304.78 576495.7539
```

And, I need the cumulative deviation new column in this DF to look like this:

```
Cum Dev
0.122993392
0.160154637
0.159888559
0.221628609
0.187604073
0.178089327
0.16687643
0.152866293
0.129326033
0.114260993
0.124487107
0.128058305
```

In Excel, the calculation would look like this with data in Excel columns Z3:Z14, AA3:AA14 for the first row: =SUM(Z$3:Z3)/SUM(AA$3:AA3)-1 and for the next row: =SUM(Z$3:Z4)/SUM(AA$3:AA4)-1 and for the next as follows with the last row looking like this in the Excel example: =SUM(Z$3:Z14)/SUM(AA$3:AA14)-1

Thank you kindly for your help,

### >Solution :

You can divide the cumulative sums of those 2 columns element-wise, and then subtract 1 at the end:

```
>>> (df["Curr Yr"].cumsum() / df["LT Avg"].cumsum()) - 1
0 0.122993
1 0.160155
2 0.159889
3 0.221629
4 0.187604
5 0.178089
6 0.166876
7 0.152866
8 0.129326
9 0.114261
10 0.124487
11 0.128058
dtype: float64
```