I have a column named BsmntQual that gives a ranking on the height of the basement per each house. These are all of the unique values in this column:


>>> ['Gd', 'TA', 'Ex', nan, 'Fa']

This is the legend of this particular column:

BsmtQual: Evaluates the height of the basement

       Ex   Excellent (100+ inches) 
       Gd   Good (90-99 inches)
       TA   Typical (80-89 inches)
       Fa   Fair (70-79 inches)
       Po   Poor (<70 inches
       NA   No Basement < Not to be confused with the nan value above

This is what I did for my other ranked columns but this one did not have NaN values:

train['ExterQual'] = train['ExterQual'].replace(['Ex', 'Gd', 'TA', 'Fa'], [4, 3, 2, 1])  # Exterior Quality

For numerical values, a common way is to fill all NaN values with the mean of the column. But what is a good way of replacing the NaN values for columns such as these?

Here is the full dataset


2 Answers 2


Your legend clearly states that missing values mean that there is no basement. You could fill the missing values with ’NoBase’ to make that point clearer (train[‘BsmtQual’].fillna(‘NoBase’, inplace=True)). When you rank them then, you just add another ranking for ‘NoBase’, maybe 0, based on your example rating that gives a higher value to more quality.

  • $\begingroup$ Read it again. The legend clearly states that NA = No Basement. If you look above that, I got a NaN value. They're not the same thing. $\endgroup$ Jun 30, 2019 at 10:22
  • $\begingroup$ @AndrosAdrianopolos If they’re not the same thing, then I’d suggest asking whoever gave you the dataset to make it clear what the missing values represent. $\endgroup$
    – m13op22
    Jul 1, 2019 at 3:10
  • $\begingroup$ Okay fine but it's pretty clear mate. NA & nan don't look that similar. Anyways. Do you have a solution to this problem? $\endgroup$ Jul 1, 2019 at 3:50
  • 1
    $\begingroup$ @AndrosAdrianopolos - when you use pd.read_csv the default parser automatically transforms the string 'NA' into nan values so this answer is correct. - there are no 'NA' values in your print.. $\endgroup$
    – yoav_aaa
    Jul 1, 2019 at 13:09
  • $\begingroup$ @yoav_aaa Good catch. Andros, if you want to ignore that behavior, you can set keep_default_na = False to keep pandas from reading NA as nan, and it will just leave those values as the string 'NA'. $\endgroup$
    – m13op22
    Jul 1, 2019 at 13:36

method : {‘backfill’, ‘bfill’, ‘pad’, ‘ffill’, None}, default None Method to use for filling holes in reindexed Series pad / ffill: propagate last valid observation forward to next valid backfill / bfill: use NEXT valid observation to fill gap

from pandas fillna, these methods can be used or fill in with the highest frequency category.


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