If you have a blog and hoping to earn some money through it then the sales are the scale to measure how well your blog is working.
Then chat if there are no direct sales and leads are not in the overall plan of your blog? In that case what would be the scale to measure the success of the blog? Some of the bloggers out there measure the success of blogs with the amount of traffic that the blog gets, some do it with the subscribers and there are others who feel that the number of comments or responses determine their blog's success.
So it boils down to a point that the success of your blog is entirely what you want out of it.
Traffic: this is the most common way to determine the success of the blog.
The traffic.
To measure the traffic each and every tool may come up with a different answer which entirely depends on the method used to calculate.
So do not expect two different tools of calculation to give you the same answer.
Use your favorite tool and check the whether there is an actual increase in the traffic than worrying about the actual numbers that are arrived at.
They may or may not be accurate.
Unique visitors: this is the figure of the number of visitors visiting your blog.
There is actually only one way to know the exact number of people coming on the blog and that is through the log in panel.
There are other techniques employed to roughly determine the number of people visiting the blog.
It is to count the total number of ip addresses or getting the cookies recorded.
I still feel that no method can give the exact or the most accurate answer.
This is because several systems may have a single ip address if they are in a single network.
And regarding cookies, half of the people get them deleted using a software or even manually.
The next issue is that many of the readers may prefer to accept the content o the blog in the form of feeds rather than visiting your blog.
In this case or if this happens then your actual number of visitors or readers is more than what is shown in the statistics.
So this way there are some methods to track the success of the blog but prefer to track the increase than on the actual numbers.