[Roberto El Duque]

I have read a few articles and listened to a few podcasts where authors are talking about their read-through rates for books in a series.

My question is: Where do the figures come from?
I have yet to publish a series, so don't know if these stats show up on KDP so...

When you hear these figures mentioned, are they what I would call a "real" read-through rate i.e. 65% of people who read book 1 actually purchased book 2....

...or are they approximated read through rates e.g. Sales of Book 2 equal 65% of Book 1 sales, and an assumption is made that all people purchasing Book 2 have already purchased Book 1?

 

[Kathelm]

or are they approximated read through rates e.g. Sales of Book 2 equal 65% of Book 1 sales, and an assumption is made that all people purchasing Book 2 have already purchased Book 1?

This is how they are calculating it. You have to do it manually by tracking total sales for each book and taking the ratio.

 

[cinisajoy]

Read through rates are the approximate percentage of people that bought book 2.  It is a reasonable assumption they have read book one.
Now usually on those books, the percentages go up with each book.  A good example would be from book 1 to book 2 is 50%.  From book 2 to book 3 is usually higher.
Let's say 100 people got book 1.  50 people went on to get book 2. Then roughly 75% of the 50 buy book 3, that is 37.5 people.    Rounding up to 38  people about 95% of them will go on to buy book 4.  That is 36 people. 
By that point those 36 people will more than likely buy the entire series.
Now the thing is if you get more and more out there with a series you will lose readers.

There is an author that her main character went from good girl to OMG this needs to be in  a brown paper wrapper behind the counter.   
She has lost many readers.  Some stopped at book 5, others 10 and a few made it to 15 before quitting.
Book 15 if I remember right was almost entirely an orgy.