hard to understand vs superficial

hard to understand

superficial

adj
  • Not thorough, deep, or complete; concerned only with the obvious or apparent. 

  • Lacking depth of character or understanding; lacking substance or significance. 

  • Existing, occurring, or located on the surface. 

  • Appearing to be true or real only until examined more closely. 

  • Situated or occurring on the skin or immediately beneath it. 

  • Two-dimensional; drawn on a flat surface. 

  • Denoting a quantity of a material expressed in terms of area covered rather than linear dimension or volume. 

noun
  • A surface detail. 

How often have the words hard to understand and superficial occurred in a corpus of books? (source: Google Ngram Viewer )