In a previous blog, we looked at some statistics on sales
for one particular popular book series, Patrick O’Brian’s Aubrey/Maturin
series, a historical fiction series about the Royal Navy during the era of the
Napoleonic Wars. Now, we will extend
this analysis, adding nine more of the most popular series in recent
history. The particular book series were
selected from a wiki article, “List of bestselling books”.
The author, series title and total sales (copies) are shown
below:
Author and Series

Total

J.K. Rowling  Harry Potter

447,000,000

Dan Brown  Robert Langdon

200,000,000

Stephanie Myers – Twilight

120,000,000

Suzanne Collins  Hunger Games

50,000,000

Robert Jordan  Wheel of Time

44,000,000

Stephen King  The Dark Tower

30,000,000

G.R.R. Martin  Game of Thrones

24,000,000

Veronica Roth – Divergent

20,000,000

Douglas Adams  Hitchhikers Guide

16,000,000

Patrick O'Brian  Aubrey/Maturin

4,000,000

Book Num  GR Reviews  GR Ratings  Sales (Wiki)  First Pub  GR Rating 
1  40,793  2,580,696  107,000,000  1997  4.38 
2  16,682  1,177,363  60,000,000  1998  4.28 
3  18,824  1,219,695  55,000,000  1999  4.46 
4  16,610  1,182,736  55,000,000  1999  4.46 
5  15,789  1,136,636  55,000,000  2003  4.40 
6  15,599  1,136,725  65,000,000  2005  4.48 
7  37,191  1,175,133  50,000,000  2006  4.57 
Total  161,488  9,608,984  447,000,000  4.43 
As the table and graph indicate, the numbers of copies sold
correlates pretty closely to the number of people who rated the books on
Goodreads, once we have normalized the data.
We do that by defining the value of the statistic as 100 for Book 1,
then comparing the following volumes to that index. For example, Volume 1 (Philosopher’s Stone)
sold 107 million copies, while volume 2 (Chamber of Secrets) sold 60 million
copies. Then 60/107 = .56, so Volume 2
is given the value 56, compared to Volume 1, which is given the value 100. Similarly for the other books and measures. The correlation coefficient between copies
sold and number of ratings is .964, which is high. Note that a value of 1.00 would indicate a
perfect correlation between two variables.
Another way to see this is to divide the number Goodreads
ratings into the number of books sold for each volume. As you can see, the number is consistently
close to 2 percent. That also shows that
Goodreads has a pretty wide reach among readers, at least as far as the Harry
Potter series is concerned:
Book Num

Title (Harry Potter and the…)

Ratings pct of Sales

1

Philosopher's Stone

2.4%

2

Chamber of Secrets

2.0%

3

Prisoner of Azkaban

2.2%

4

Goblet of Fire

2.2%

5

Order of the Phoenix

2.1%

6

HalfBlood Prince

1.7%

7

Deathly Hallows

2.4%

Now let’s look at the book series in detail, focusing on the
number of Goodreaders rankers vs the position of the books within the
series. We will go by series book sales,
largest to smallest.
1 – Harry Potter
(J.K. Rowling)
We see that the series followed a power law quite closely,
with the second and the last books departing somewhat from the best fit
curve. The median book had about half
the raters that the first book had. As
noted above, if we divide Goodreads raters into copies sold, we come up with a
figure of 2.1%. This relatively low
figure may be a reflection of the fact that a substantial part of the audience
did not participate on Goodreads, perhaps because they were too young.
2 – Robert Langdon
(Dan Brown)
In this case, we see that the function departs from the
power law form by quite a bit. That’s
mostly because the second book of the series, The Da Vince Code was really the
big breakout success. In fact, most
people think it is actually the first book in the series, which was actually
Angels and Demons. But the second book
caught the public’s fancy more, probably because of the implications for the
church. Note that the last two books seem to have lagged the first two quite
badly, relative to the first two, at least in this data. But it is hard to repeat that level of
success. If we divide the number of
Goodreads raters into the number of copies sold, we come up with a ratio of
1.4%. This probably reflects an older,
less socialmedia driven audience for this type of a series (and perhaps a less
enthusiastic one).
3 – Twilight
(Stephanie Myers)
This four book series followed a relatively flat power law
very closely. After the initial dropoff
from Book 1, she seems to have held on to about 40% of the initial book raters,
very consistently. If we divide the
number of Goodreads raters into the number of copies sold, we come up with a
ratio of 4.2%, a middlinghigh figure.
4 –Hunger Games
(Suzanne Collins)
This series also conformed closely to the power law, though
naturally that’s easier to do with only three data points to fit. She also did a very good job of holding onto
about half of the raters through the final two books of the trilogy. If we divide the number of Goodreads raters
into the number of copies sold, we come up with a ratio of 10.4%. This would seem to indicate that readers of
this series were very enthusiastic about sharing their ratings of the book and
were very social media aware.
This series conformed fairly well to the power series, but
with some bumps along the way. From
reading reviews, it seems that the series lagged somewhat in the latter middle
part, then picked up again towards the end.
Nonetheless, it did an excellent job of holding onto raters as the
series progressed, given its length.
Nearly half were still engaged for most of the latter half of a long
series. If we divide the number of
Goodreads raters into the number of copies sold, we come up with a ratio of
2.2%. Due to the length of the series,
this might also reflect an older, less socialmedia driven audience.
6 –The Dark Tower
(Stephen King)
This one is almost a textbook perfect example of a nice
power law. King did pretty well to hold
on to a lot of raters over a long series as well. If we divide the number of Goodreads raters
into the number of copies sold, we come up with a ratio of 2.1%. Again, due to the length of the series, this
might also reflect an older, less socialmedia driven audience.
7 –Game of Thrones
(G.R.R. Martin)
Yes, I know that’s not the real name, but that’s the name of
the TV show, so I figure that’s how most people think of it. Again, it is almost a pictureperfect example
of a power law. The last book has lagged
a bit, but he still has two more books to go.
Again, he has done a good job of holding on to nearly half of his
original audience, as inferred from Goodreads raters. If we divide the number of Goodreads raters
into the number of copies sold, we come up with a ratio of 7.6%. Perhaps this is at least partially due to the
series having a concurrent TV spinoff, with the consequent buzz and cross
promotion.
8 –Divergent
(Veronica Roth)
This is a pretty decent fit, but there are only three points
to fit, so that has to be borne in mind.
If we divide the number of Goodreads raters into the number of copies
sold, we come up with a ratio of 8.2%.
As with the Twilight series, this would seem to indicate an audience
that is very enthusiastic about the books and keen to share their feelings on
social media.
9 –Hitchhikers’ Guide (Douglas Adams)
Again, this is a nearly perfect fit to a power law. However, it has quite a steep dropoff, with
the first book in the series getting far more ratings than the earlier
books. This seems to be a feature of
older books and how they interact with Goodreads. It may be that it is more a reflection of
people’s recall of an older series, rather than being related to underlying
sales. However, if we divide the number
of Goodreads raters into the number of copies sold, we come up with a ratio of
6.1%, which is quite high for a series whose author died quite a while back and
whose audience probably skews older.
10 –
Aubrey/Maturin (Patrick O’Brian)
Again, this is a very good fit to a power law, especially
given the length of the series. We see a
bump at Book 10 (that was the book that shared the title with the movie “The
Far Side of the World”). Book 2 is also
a bit low. If we divide the number of
Goodreads raters into the number of copies sold, we come up with a ratio of
2.9%, which is what we might expect for a series whose author died quite a
while back and whose audience probably skews older.
Some Conclusions
·
It does appear that the number of Goodreads
raters reflects the number of copies sold fairly accurately within a series
(i.e. there is a good correlation). At
any rate, that appears to be the case for the Harry Potter series. For that series, the number of Goodreads
raters was about 2% of the copies that were sold.
·
However, a similar calculation went from a high
of 10% for the Hunger Games series to a low of 1.4% for the Dan Brown
series. So, there is some considerable
variability in different audiences to make their way to Goodreads and to share
their opinions, via rating the books.
·
Nearly all of these very popular series fit a
power law very well. The main exception
was the Dan Brown series, in which The Da Vinci Code was the exception to the
rule. But that book truly was
exceptional, on a lot of grounds.
·
Most of the newer series managed to have 40% to
50% of the Goodreads raters involved by the midpoint of the series, relative to
the first book. The older series had a
much greater rate of dropoff, though this may also be related to the fit between
the audience of the series and the members of Goodreads.
Next time we will look at whether these findings hold for
Amazon reviews, or whether things are different in the Kindle world.
Note that I will put up the raw data that these graphs are based on a little later in the week.
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