ARMA model order selection using arma_order_select_ic from statsmodel - python-3.x

Question

I am using the arma_order_select_ic from the statsmodel library to calculate the (p,q) order for the ARMA model, I am using for loop to loop over the different companies that are in each column of the data-frame. The code is as follows:
import pandas as pd
from statsmodels.tsa.stattools import arma_order_select_ic
df = pd.read_csv("Adjusted_Log_Returns.csv", index_col = 'Date').dropna()
main_df = pd.DataFrame()
for i in range(146):
order_selection = arma_order_select_ic(df.iloc[i].values, max_ar = 4,
max_ma = 2, ic = "aic")
ticker = [df.columns[i]]
df_aic_min = pd.DataFrame([order_selection["aic_min_order"]], index =
ticker)
main_df = main_df.append(df_aic_min)
main_df.to_csv("aic_min_orders.csv")
The code runs fine and I get all the results in the csv file at the end but the thing thats confusing me is that when I compute the (p,q) outside the for loop for a single company then I get different results
order_selection = arma_order_select_ic(df["ABL"].values, max_ar = 4,
max_ma = 2, ic = "aic")
The order for the company ABL is (1,1) when computed in the for loop while its (4,1) when computed outside of it.
So my question is what am I doing wrong or why is it like this? Any help would be appreciated.
Thanks in Advance

Answer 1

It's pretty clear from your code that you're trying to find the parameters for an ARMA model on the columns' data, but it's not what the code is doing: you're finding in the loop the parameters for the rows.
Consider this:
import pandas as pd
df = pd.DataFrame({'a': [3, 4]})
>>> df.iloc[0]
a 3
Name: 0, dtype: int64
>>> df['a']
0 3
1 4
Name: a, dtype: int64
You should probably change your code to
for c in df.columns:
order_selection = arma_order_select_ic(df[c].values, max_ar = 4,
max_ma = 2, ic = "aic")
ticker = [c]