Chunky vs. Smooth Strings
CJam, 19 bytes

q_1&gt;_@.=:!1b\,d/4mO


100% chunky source code that calculates chunkiness.

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How it works

q_                  e# Read from STDIN and push a copy.
  1&gt;_               e# Discard the first character and push a copy.
     @              e# Rotate the original on top.
      .=            e# Vectorized comparison (1 if equal, 0 if not).
        :!          e# Mapped logical NOT (0 if equal, 1 if not).
          1b        e# Base 1 conversion (sum).
            \,      e# Swap with the shortened string and push its length.
              d/    e# Cast to double and divide.
                4mO e# Round to four decimal places.


Obviously, NaN rounded to 4 decimal places is 0.