case...when or Hash#fetch
I would have just gone for an if/else, this really isn't a good use case for a case statement and I've never seen anyone use a hash like that except for demonstration purposes:

def choose(value)
  if value &lt; LOWER_LIMIT
    'a'
  elsif value &lt;= UPPER_LIMIT
    'c'
  else
    'b'
 end
end


or possibly using a ternary

def choose(value)
  value &lt; LOWER_LIMIT ? 'a' : value &lt;= UPPER_LIMIT ? 'c' : 'b'
end


note that I find it easier to keep my conditions in ascending order.

If this is a common pattern I would consider extending ranges.

class Range
  def case(value, less, in, more)
    if value &lt; first
      less
    elsif include?(value)
       in
    else
       more
    end
  end
end

(LOWER_LIMIT..UPPER_LIMIT).case(value, 'a', 'c', 'b')