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I have a datset with Scores and Categories and I would like to calculate the Standard Deviation of these scores, per category. The data look something like this:

Category    Score    
AAAA        1
AAAA        3
AAAA        1
BBBB        1
BBBB        100
BBBB        159
CCCC        -10
CCCC        9

What I would then like is the Standard Deviation of each Category. I know that with numpy I can use the following:

numpy.std(a)

But the example I can find only have this relating to a list and not a range of different categories in a DataFame.

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  • $\begingroup$ I highly recommend you to use pandas in these types of work, as the answer suggested. $\endgroup$ – Blaszard Aug 14 '17 at 11:23
  • $\begingroup$ This one should be moved to stack-overflow. There's no science stuff here. $\endgroup$ – Piotr Rarus - Reinstate Monica Nov 25 '19 at 14:38
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You can easily do this using pandas:

import pandas as pd
import numpy as np

df = pd.DataFrame([["AA", 1], ["AA", 3], ["BB", 3], ["CC", 5], ["BB", 2], ["AA", -1]])
df.columns = ["Category", "Score"]
print df.groupby("Category").apply(np.std)
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  • $\begingroup$ Amazing. Great answer! $\endgroup$ – Taylrl Aug 14 '17 at 13:33
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    $\begingroup$ I think we can get rid of .apply() - df.groupby("Category").std(ddof=0) $\endgroup$ – MaxU Aug 15 '17 at 12:08
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I have a slight variation in the input data. I have more than one column, so how to give command to pick a specific column for the calculation of std deviation.

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  • $\begingroup$ adapting @MaxU's comment, you can do column access on the groupby object as if it was a dataframe. So df.groupby('Category')['specific_column'].std(ddof=0) would work $\endgroup$ – Tom M. Nov 25 '19 at 15:01

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