calculate standard deviation pandas dataframe

336
March 17, 2018, at 9:26 PM

I am using norway_new_car_sales_by_model.csv Dataset here dataset which you find. I want to find the model that has highest sales fluctuation over the years. I am using standard deviation of the yearly total sales for each model. Expected output is:

import pandas as pd
import numpy as np
 data=pd.read_csv("norway_new_car_sales_by_model.csv",header=None,encoding="latin-1")
 data.columns = ['Year','Month','Make','Model','Quantity','Pct']#give column name
 data.drop(data.head(1).index, inplace=True) #drop first row
 data[['Quantity']]=data[['Quantity']].astype(np.int64)
 data.dropna(subset=['Quantity'], how='all', inplace = True)

 maketotal_1 = data.pivot_table(values='Quantity',index=['Month','Model','Make'],aggfunc=np.std)

My question are that

1) I did not handle nan values... even if i try many codes...

2) How to get Audi A4 Audi from Index column

Answer 1

I think need:

First remove parameter header=None from read_csv, because first in csv are columns names:

data=pd.read_csv("norway_new_car_sales_by_model.csv",encoding="latin-1")
print (data.head())
   Year  Month         Make              Model  Quantity   Pct
0  2007      1  Volkswagen   Volkswagen Passat      1267  10.0
1  2007      1      Toyota         Toyota Rav4       819   6.5
2  2007      1      Toyota      Toyota Avensis       787   6.2
3  2007      1  Volkswagen     Volkswagen Golf       720   5.7
4  2007      1      Toyota      Toyota Corolla       691   5.4

Apply pivot_table function with np.std:

maketotal_1=data.pivot_table(values='Quantity',index=['Month','Model','Make'],aggfunc=np.std)
print (maketotal_1.head())
                          Quantity
Month Model       Make            
1     Audi A3     Audi   50.986109
      Audi A4     Audi   60.549704
      Audi A6     Audi         NaN
      Audi Q3     Audi         NaN
      BMW 2-serie BMW          NaN

Last first remove NaNs by dropna and use reset_index for convert MultiIndex to columns and create unique default index:

df1 = maketotal_1.dropna().reset_index()

Last per groups by Make get indices of max values by idxmax and then select rows by loc:

df3 = df1.loc[df1.groupby('Make')['Quantity'].idxmax()]
print (df3)
     Month              Model            Make    Quantity
447     12            Audi A3           Audi   119.867427
415     11             BMW i3            BMW   460.936366
56       2        Ford Mondeo           Ford   169.889880
235      6         Honda CR-V          Honda   171.579671
457     12       Hyundai ix35        Hyundai    32.526912
348      9       Kia Sportage            Kia    55.154329
60       2         Mazda CX-5          Mazda   144.030957
14       1  Mercedes-Benz GLC  Mercedes-Benz   119.501046
160      4     Mitsubishi ASX     Mitsubishi   312.541197
391     10        Nissan Leaf         Nissan   225.322584
114      3         Opel Astra           Opel    85.182158
22       1        Peugeot 207        Peugeot    97.962578
168      4        Renault Zoe        Renault    53.740115
395     10      Skoda Octavia          Skoda   121.668767
122      3      Suzuki Vitara         Suzuki    85.559921
123      3      Tesla Model S          Tesla   510.400823
33       1     Toyota Corolla         Toyota   326.683333
179      4    Volkswagen Golf     Volkswagen   454.872681
485     12          Volvo V40          Volvo   183.919366

EDIT:

There is no Citroen because np.std return NaN:

print (maketotal_1[maketotal_1.index.get_level_values('Make') == 'Citroen '])
                                    Quantity
Month Model               Make              
11    Citroen C4 Aircross Citroen        NaN
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