new_contents: import pandas as pd
  
  
  def standarize(data):
      ''' Return correlated data of each input.'''
      return (data - data.mean()) / data.std()
  
  
  def clean_std(df1, df2, key1, key2, key3):
      ''' Return new cleaned and correlated dataframe
          of the two input dataframes.'''
      # merge input dataframes
      stats = df1.merge(df2, on=key1, how='inner')
      # create new dataframe with the three input keys as columns
      stats_clear = pd.DataFrame(data={key1: stats[key1].values,
                                       key2: stats[key2].values,
                                       key3: stats[key3].values
                                       })
      # update dataframe with mean values
      stats_clear_mean = stats_clear.groupby(key1, as_index=False).mean()
      # remove key1 column so to keep only numerical data
      del_id = stats_clear_mean.drop(key1, 1)
      # use apply method to return correlated dataframe
      stats_std = del_id.apply(standarize)
      return stats_std
  
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