How to find outliers within groups in a dataframe

Question

I have a df which looks like the following:

Group.	Score.
red	34
blue	42
green	1000
green	34
blue	34
red	42

I would like to add a column onto this which specifies if the value is an outlier. If there were no groups then I would use something like:

df['outliers'] = df[df[col] > df[col].mean() + 3 * df[col].std()]

But how would I do this so it is within the groups?

Answer 1

You can use GroupBy.transform :

df["is_outlier"] = df.groupby("Group.").transform(lambda x: (x - x.mean()).abs() > 3*x.std())

In each group, we take the distance of elements from the group mean and see if its absolute value exceeds 3 times std of the group.