code: def root_mse(arr, axis=0):
      return np.sqrt(np.square(arr).sum(axis=axis) / arr.shape[axis])
  
imports: import numpy
  import pytest
  from nipy.io.api import load_image
  from nipy.testing import assert_almost_equal, assert_array_almost_equal, funcfile
  from ..pca import pca
label_desc: Use image node_id 3 for calling a global function with example usage: root_mse(arr, axis) and returns: np