What would you consider to be the most accurate keyword research tool when it comes to keyword volume? I've been using the Google Keyword Planner to get a rough idea on the volume of keywords, but some of the other keyword research tools come up with keyword volumes that aren't even close to the planner. For example, a keyword in GKP shows a volume of 14,800 while SEM Rush shows a volume of 50. There are a number of keywords that come up that way when I'm searching. Another one shows 18,100 in GKP but it only shows 1,800 in SEM Rush. Why is their such a dramatic difference in volume between the two?