Normalized RMSE & Normalized MAE

To compare model performance across different datasets by scaling the error metrics, you can normalize both RMSE and MAE:

Normalized RMSE:

• using the mean of data set: $NRMSE=\frac{RMSE}{\overline{y}}$ (i.e., Coefficient of Variation of RMSE)

• using the difference between maximum and minimum in data set: $NRMSE=\frac{RMSE}{y_{max}-y_{min}}$,

• using the standard deviation of data set: $NRMSE=\frac{RMSE}{\sigma }$, or

• using the interquartile range of data set; $NRMSE=\frac{RMSE}{Q1-Q3}$, i.e. the difference between 25th and 75th percentile, of observations.

https://www.marinedatascience.co/blog/2019/01/07/normalizing-the-rmse/

Normalized MAE: 

• aka. Coefficient of Variation of MAE; Coefficient of Variation (CV) =  the ratio of the standard deviation ${\displaystyle \sigma }$ to the mean ${\displaystyle \mu }$, ${\displaystyle CV=\frac{\sigma }{\mu }.}$

• using the mean of data set: $N\mathrm{MA}E=\frac{\mathrm{MA}E}{\overline{y}}$ 

https://d2uars7xkdmztq.cloudfront.net/app_resources/38997/documentation/135539_en.pdf