# Understanding MASE and sktime implementation

Sorry, but after reading the Hyndman's paper here I don't understand a couple of key points.

• The term "in-sample"
• The implementation in Python using sktime

Regarding the first point, I assume that "in-sample" means the "testing" set.

On the other hand, I have a time series for which I computed the residuals ie $$e_{i}=y(t)-\hat{y}(t)$$ (where $$\hat{y}(t)$$ stands for the prediction or forecast). I store the TEST set real values and the residuals on a Panda's DataFrame called "pred", like this:

pred = pd.DataFrame({"Date":[599529600000,599616000000,599702400000,599788800000,599875200000],"Temp":[14.45,14.3,17.4,18.5,16.8],"AR_predicted":[14.2805985725,14.4314555264,14.4887198437,14.4663198212,14.4440888856],"AR_residuals":[0.1694014275,-0.1314555264,2.9112801563,4.0336801788,2.3559111144]})


Then, when I compute the MASE manually using the code below:

pred['1difference'] = np.abs(pred['Temp'] - pred['Temp'].shift(1))
quotient = pred['1difference'].mean()
pred['q'] = np.abs(pred['AR_residuals'])/quotient
MASE = pred['q'].mean()
print("The MASE of the final model is: {:.2f}".format(MASE))


prints: 1.69

However, if I use the MeanAbsoluteScaledError from sktime Python library as follows:

mean_absolute_scaled_error(y_true=pred['Temp'], y_pred=pred['AR_predicted'], y_train=y_train)


prints: 1.52

Where it comes this difference and why I need to place y_train=y_train if I only want the MASE of the TEST set?

Thank you very much.

• "In-sample" refers to the training set, i.e., the data on which you fit your time series model. (This detail is frequently gotten wrong. Honestly, you can of course use a different denominator than the MAE of in-sample 1-step forecasts, but then this should be noted, but it rarely is.) Regarding your second question, can you please edit your question to include a Minimal Working Example? Sep 21, 2022 at 6:03
• First of all, thank you very much for spending some time reading my question @StephanKolassa. Now, I see what is the source of the discrepancy. I was doing all the calculations in the TEST set instead of using the TRAIN set. I'm going to redo the calculations using the TRAIN set to see if the manual solution matches the one provided by the function. Sep 21, 2022 at 8:49

I figured out the problem. The right formula is:

$$MASE = \frac{MAE_{train,forecast}}{MAE_{train,Naïve}}$$

then in Python my code should be:

results['1difference'] = np.abs(results["Temp"]-results["Temp"].shift(-1))
quotient = results[results['Set']=="Train"]['1difference'].mean()
results['q'] = np.abs(results['Forecast_Residuals'])/quotient
MASE = results[results["Set"]=="Train"]['q'].mean()


which explains why the sktime function mean_absolute_scaled_error() asks for the y_train.