The Google toolbar can be used to see the rank that Google gives a website. Google update this information about 3 or 4 times a year.
Matt Cutts explains the reason for this is that people get hung up on page rank and the goal is to make people focus on making websites better in terms of content.
How can you identify the cause of a page rank drop?
If an external link was adding power and has now been dropped then this will cause this. Or if you have changed the internal linking structure of your website then this will also have an effect.
Page ranking dropping by 30% is often due to breaking guidelines and this becomes detected and then this power will be removed. Remove the paid links and then ask Google to check and then the page rank will return.
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