How to calculate RMSE, skewness, and kurtosis of raster image?

I'm doing a project to analyze the impact different interpolation techniques plays on DEM accuracy.

In order to do so, I need to calculate the RMSE, skewness, and kurtosis.

I have:

1. an original source DEM,


2. a DEM generated from 10 different interpolation techniques,


3. a DEM generated by the subtraction of above two DEMs.

How do I calculate the three parameters?