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Some people believe that the most reliable way to determine keyword difficulty is to visit the SERPs and see who holds top positions and then you will know "who you must defeat". If you want something a bit more reliable, then click into the top-ranking pages, examine them and then know "what you must defeat". The combination of who and what is very informative. Who and what have informed in advance of battles for thousands of years over all types of terrain. I don't use keyword difficulty scores.  I want the information above.  Then, I match that against my skills and the resources required to become competitive. Great mistakes are made when skillful people allow keyword scores to scare them away from competition and when low keyword scores lure less skillful people into battles above their abilities.