These days, a lot of writers are doing series. There are good reasons for that  once you
have built up an audience for a certain setting, cast of characters and genre,
you would like to maintain that audience.
It seems natural that a series would be the way to go. But what might you actually expect from a
series? For example, how many people
will move on from book 1 to book 2, book 2 to book 3 and so on? It seems likely that you will lose some
people along the way, but is there a pattern to that? To get a feel for this, let’s look at some
results for some well known long running book series. Naturally, we can only look at a few “ideal
type” cases, but with luck that will give us some insights that are typical for
most series.
First, we will look at Patrick O’Brian’s Aubrey/Maturin
historical fiction series. That is the
series of books that the recent (released in 2003) movie “The Far Side of the World”, featuring Russel Crowe, was based
on. They are about a Royal Navy captain
and a ship’s doctor/spy, set during the era of the Napoleonic wars, roughly
1800 to 1815. Why did I pick this series
first?
·
I have read them all, so I have a good sense of
how the series evolved.
·
It’s a long series (20 books) so it really tests
the idea of loyalty to a series.
·
It has sold a lot of books, and has had a lot of
fans, so the statistical power of the analysis should be high (that just means
that the numbers are big, so the results are probably grounded in some
underlying realities, not random noise).
·
It has spanned the era of print books sales in book
stores to the era of ebooks sales in online stores, so we might be able to see
whether the change in how stories are stocked and delivered has affected how
people consume series.
To begin, we will look at how the
series did via a number of Goodreads
measures. As you may know, Goodreads is a site where readers can leave reviews, ratings, and recomendations of the books they have read. The measures that we will look at are Numbers of
Reviews, Numbers of Raters, Average Rating, and Number of Editions. The results, as taken from the Goodreads
website are shown below. The year that
the book was first published is also shown, to give some idea of the time scale
involved. There’s a reason the first
four books are highlighted, which we will get to later.
Book Num

Title

GR Reviews

GR Ratings

GR Rating

Editions

First Pub
 
1

Master and Commander

1,700

21,161

4.08

90

1969
 
2

Post Captain

459

8,760

4.29

63

1972
 
3

HMS Surprise

319

7,879

4.40

54

1973
 
4

The Mauritius Command

224

6,885

4.32

53

1977
 
5

Desolation Island

233

6,307

4.35

50

1977
 
6

The Fortune of War

170

5,855

4.35

43

1978
 
7

The Surgeon's Mate

152

5,575

4.35

40

1980
 
8

Treason's Harbour

110

5,217

4.35

39

1980
 
9

The Ionian Mission

137

4,390

4.28

41

1981
 
10

The Far Side of the World

156

5,473

4.41

48

1984
 
11

The Reverse of the Medal

126

4,221

4.38

40

1986
 
12

The Letter of Marque

119

4,772

4.43

36

1988
 
13

The Thirteen Gun Salute

125

3,920

4.35

36

1989
 
14

The Nutmeg of Consolation

114

4,113

4.37

39

1991
 
15

Clarissa Oakes/The Truelove

107

3,778

4.33

35

1992
 
16

The WineDark Sea

102

3,709

4.36

34

1993
 
17

The Commodore

99

3,626

4.37

38

1994
 
18

The Yellow Admiral

100

3,813

4.32

36

1996
 
19

The Hundred Days

93

3,327

4.31

32

1998
 
20

Blue at the Mizzen

128

3,213

4.34

41

1999
 
4,773

115,994

4.34

888

As you can see, for most measures
there was a fairly steady decline from Book 1 to Book 20, though some books in
the latter part of the series seem to have done better than the book that
immediately preceded it  in mathematics, we would say that it is not a
monotonic series, but in statistics we might say that it comes pretty close to
one (it is quite well modelled by a power law, in fact).
The data is graphed above, with the various measures (Number
of Editions, Number of Goodreads Ratings, and number of Goodreads Reviews)
scaled in such a way that the measures for the first book are assigned the
value of 100, and the measures for books after that are assigned numbers
proportional to that initial value. So,
for example, the first book had 90 editions printed, while the second book had
63 books printed. In our scaled variable
we have assigned 100 to the first book, and 70 to the second book (63/90 =
0.70, so the second book is given the value 70). The reason for using these scales (it’s
called normalizing) is so that we can compare the three line graphs on the same
scale.
There are a lot of interesting results here. First off, we see that all of the graphs
decline steadily (each shows a decay curve), but they fall off at different
rates. The falloff for the number of
editions is slowest. That’s interesting,
since the number of editions is probably the measure that best tracks the
number of books sold and read. After
Book 5, the number of editions printed falls to about 40% to 50% of the number
of editions printed for the first book.
So, Patrick O’Brian appears to have held on to about half of his initial
book purchasers as the series matured.
There was an uptick at Book 10  that’s “The Far Side of the World”,
which was also the title of the movie starring Russell Crowe. So, that clearly seems to have given the book
a bounce.
There was also an uptick for the final book of the series
“Blue at the Mizzen”. A reasonable
hypothesis is that those extra editions may represent sales to people who
followed part of the series and dropped out, but who might have decided to buy
the final book to see how it turned out.
However, in some ways Book 19 was really the end of the series (Napoleon
is defeated) and book 20 could be thought of as the start of another series
that featured the same main characters in a different setting (the plot moves
from the Napoleonic wars to the wars of liberation in South America). But the author died shortly after Book 20, so
there was no chance for a “next generation” followup. So the final book uptick might be related to
people buying into a new series or it might be related to the wrapup of the
original series. We’ll never know.
There are some other interesting features of these decay
curves  first, how the decay curve of Goodreads ratings falls off more rapidly
than the decay curve of the number of editions and secondly how the line
representing the number of Goodreads reviews falls off even more sharply. So, it appears that people might be less
willing to invest the time and energy into rating or reviewing books that they
read in a series, as the series goes on.
Also, it appears that they are more willing to invest the time in a
rating than in a review. That makes
sense, as a rating only takes a few seconds, while a review can take five or
ten minutes  even much longer than that, for those who take their reviewing
very seriously indeed.
One other interesting aspect of the decay curves is that
they are well modelled by our old friend the power law, of which I have written
previously. The fitted lines next to
the jagged data lines are these power functions. The RSquared values next to the respective
lines indicate that the fits are quite robust, in a statistical sense (an
RSquare of 1.00 would indicate that the data fit the powerlaw function perfectly,
so values in the 0.85 to 0.95 range are really quite good fits.
The above data also shows that after the first few books,
the number of reviews and rankings correlated rather nicely with the number of
editions of the book. Since we assume that the number of editions
printed correlates fairly well with the number of copies sold, we can therefore have some more confidence in
the notion that the total number of reviews a book gets scales fairly well with
its total sales. This assumes, of course,
that each edition had more or less the same number of copies printed and
sold. This can be seen in the graph
below. Note, however that for the
initial books in the series, the number of reviews was higher than would be
expected from the relationship in the graph.
Again, this indicates that people may be more enthusiastic to
review/rank near the beginning of a series than later on.
It is also worth noting how the average rating of the books
went, as the series progressed. As you
can see, the first couple of books actually had the lowest rating, and after
that the ratings were quite consistent, at a bit under 4.4, for the most
part. So, it would appear that as the
series went on, the readers who dropped out were (not surprisingly) those who
were less satisfied with the books, and the readers who stayed with the series
were those who were more satisfied. So,
the audience was smaller, but more loyal as the series continued.
As we can see, the number of reviews in the Kindle store do not show the decay curve pattern that was evident in the Goodreads data, which was probably primarily based on legacy print book sales. In the Kindle store, the last two books had about as many reviews as the first two, and the others had 50 or more reviews, compared to the 85 or 90 for the top reviewed volumes. So, perhaps the always available nature of the ebooks in the Kindle store has altered the underlying sales dynamics of the series. Of course when it comes to ebook sales for books published before 2000, we are always looking at the “long tail”, so we might be seeing the dynamics of the long tail, which are generally thought to be underlain by a much flatter power law than initial book sales.
These are all good things to keep in mind when you evaluate the success of your own series, if you are a writer or publisher, especially if you are a selfpublisher or small scale publisher. To summarize:
·
There is a powerlaw like decay curve (in sales and other measurs), or at
least there was in the legacy system.
·
The slope of that curve varies depending on the
measure, with the tendency to rank or review probably falling off faster than sales, as the series
goes on.
·
If your book gets made into a movie, you will
most likely get a bump in sales J.
·
There may be a bump at the end of a long series,
as people who dropped out of some of the middle books come in to see how things
turned out.
·
Numbers of reviews and or numbers of rankings
(not average rankings) probably scale reasonably well with sales.
·
The dynamics of print book series and ebook
series may be quite different, with the ebook series possibly having a much
flatter decay curve (or none at all).
Well, that’s just one series (a highly successful one) whose
sales dynamics we have attempted to infer from Goodreads and Amazon Kindle
data, available to the public. In later
blogs we will see whether these results hold for some other series in other
genres, such as Robert Jordan’s Wheel of Time series or J.K. Rowling’s Harry
Potter series. We will also try to test
some recent ebook only series, to see if the dynamics of those are different.
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