Amazon Sales Rank, and What Math Geeks Do

Today, I asked my son’s if they would like to buy The Oatmeal’s Why Grizzly Bears Should Wear Underpants. They laughed uproariously at the title, and then Eli asked, “Is that the #1 book on Amazon?” In fact, it’s not. At the time of this writing, its ranking was #624. “That’s not #1,” Alex affirmed, then added, “but it’s a lot better than your book.”

Ha-rumph.

“A lot better” is highly subjective. Sure enough, the #3,517 ranking of Math Jokes 4 Mathy Folks has an absolute difference of 2,893 compared to WGBSWU; or, if you’re into ratios, the rank of my book is five times as much as the rank of WGBSWU. But what does that really mean?

In practical terms, it means that the number of copies of WGBSWU that will sell on Amazon this week is approximately six times the number of copies of MJ4MF that will sell during the same period. If my calculations are correct, that is. No one is really sure how ranking translates to sales, but I estimate that approximately 250 copies of MJ4MF and 1,500 copies of WGBSWU will sell this week.

This is what math geeks do: We try to understand everything quantitatively.

I took weekly sales data for MJ4MF and compared that with the book’s average ranking for the week. I randomly chose 20 weeks in 2012-13 for this analysis, because while pulling weekly sales data is relatively easy — it’s provided at Amazon Author Central — determining weekly average ranking is more difficult, since data has to be pulled day by day. And it’s not as simple as just exporting the data to Excel or a CSV file… the data is provided in a graph, and if you want to manipulate that data in any way, you have to look at each point on the graph, determine its value, and then enter it manually. Ugh.

The graph below shows the relationship between average rank and weekly sales:

The regression equation S = 914.77 × R^-0.977 gives a reasonably good fit (r = 0.89). What’s interesting is that this formula is less accurate in November and December than during the rest of year. There are two reasons for that. First, sales increase dramatically during the holiday shopping season. Second, such a formula is bound to be less accurate with larger numbers.

The average rank for December 9-15 was #3,592, and using the formula above, approximately 253 copies of MJ4MF should have sold. (I suspect that estimate is a little low. For the same week last year, the average rank was #4,573 and 277 copies were sold.)

Amazon posts sales data for each week on the following Friday. Sales data for last week won’t post until December 20. I’ll update this post on Friday and let you know how well I did.

[Update, 12/20/13: A record-breaking 335 copies of MJ4MF sold December 9-15. (Thank you!) But as predicted, the estimate was indeed low. As I gather more data, perhaps I will be able to create a better model.]

Math Fortune Cookies

Today might be Fortune Cookie Day. Hard to say, really, because there are also plenty of references on the web that claim July 20 is Fortune Cookie Day, and the good folks at Holiday Insights claim that there are references to a Fortune Cookie Day in April, May and June, too. But honestly, who cares? No one should lose sleep over an incorrect date for a made-up holiday.

Besides, if you can accept that today is Fortune Cookie Day, well, that gives me a good reason to now tell you two fortune cookie stories.

The first concerns the publication of Math Jokes 4 Mathy Folks. About an hour after Bob Reed called to tell me that he’d like to publish my book, I was dining at a Chinese restaurant. The fortune in my cookie read: Your current plans will succeed. Though I am unwilling to ascribe the success of a book to a fortune cookie, the fortune appears to have been true. Since publication on August 9, 2010, more than 5,000 copies of MJ4MF have been sold. Though I am still holding out hope that it will sell a million copies, I cannot be disappointed in a book of math jokes that reaches 5,000 people.

The second story involves my friend Andy Fielding. The day before he left for Africa to serve two years in the Peace Corps, he and I were dining at a Korean restaurant. After the meal, two fortune cookies were placed on the table. I told him to select one. “No, no, you first,” he insisted.

“But you need the good luck,” I said. “You’re leaving tomorrow.” He repeatedly refused, and the argument continued for 20 minutes. “Oh, fine!” I said finally, and took one. The fortune: You are about to take a long and safe journey. “Dammit,” I said as I showed it to Andy. “This was meant for you!”

“It’s okay,” he said as he showed me his fortune, which read: You are about to take a long and safe journey.

Someday, I hope to open a Chinese restaurant. The portions will be very large, and the existence of leftovers is guaranteed by the Chinese Remainder theorem.

When I do, I look forward to generating creative fortunes to place inside the cookies. Here are a few. (Feel free to add to this list by posting your favorite fortunes in the comments section, or get creative and write one of your own.)

• You are a complex person, and i would like to be your friend.
• When life throws you a curve, calculate the slope of the tangent at the point of inflection.
• You will live a long life. If you marry an actuary, it will feel even longer.
• Some day you will find a useful application for Ceva’s theorem. (Maybe.)
• Your lucky number is the square root of 17.
• Fame and fortune will find you… unless you lock yourself in an attic, trying to prove the Riemann Hypothesis.
• Will you still need me, will you still feed me, when I’m 2⁶?
• I have found an elegant proof of Fermat’s Last Theorem, but this fortune is too small to contain it.
• You are good at solving problems. Textbooks fear you.
• This cookie contains no fortune.
• Your students secretly agree that your head is not in proportion to your body.
• A foolish man will try to write a better fortune than this, but a mathematician will find it sufficient to know that a better fortune exists.
• When someone finds a counterexample to your proof, look for a different proof.
• A conclusion is your last thought before you got tired of thinking.
• You are so smart that you do not need answer keys.
• The fortune of this cookie is obvious.
• You are good at geometry. Q.E.D.
• Greet new friends with a handshake. At a math social, greet new friends with the handshake problem.
• Do not follow the instructions in this fortune cookie.
• Do not kiss a mathematician on the lips. Ever.

Coming Up Through the Ranks

Through some special features at Amazon Author Central, I am able to know the daily sales rank of Math Jokes 4 Mathy Folks. My sales rank at the end of each of the last three days was 44,404, 96,990, and 35,355, respectively. I thought that was interesting — three days in a row when the sales rank was a five-digit number in which one digit occurred at least three times. What’s the likelihood of that? Stated more formally:

Assuming that the sales rank of MJ4MF is always a five-digit number, what is the probability that three consecutive days’ sales ranks will contain a digit that occurs in the sales rank at least three times?

The sales rank of MJ4MF has never been a five-digit number in which the same digit is repeated five times. (Bummer!) The probability of that occurrence, though, is even less likely than the situation described above — though I won’t tell you exactly how much less likely, so as not to spoil your fun!

In Bookstores Now

Here’s a math problem for today:

The sum of three consecutive integers is 27. What is the product of the integers?

Today is August 9, 2010, also known as 8/9/10. It’s no coincidence that today was chosen as the official publication date for Math Jokes 4 Math Folks. 

You can preview the first chapter on the NCTM web site.

You can also order a copy online at the following online retailers.

• Amazon
• Barnes and Noble
• National Council of Teachers of Mathematics
• Robert D. Reed Publishers

Thanks to those of you who purchased a pre-publication copy. To those of you who buy a copy in the future — thanks in advance!

Book Signing in Colorado Springs – Sat, June 26

Will you or someone you know be in Colorado Springs this weekend? So will I! I’m doing a book signing at Borders Southgate. Details below.

Booksigning at Southgate Borders – Math Jokes 4 Mathy Folks
Why is 6 afraid of 7? How many mathematicians does it take to screw in a light bulb? What type of lingerie does a mermaid wear to math class? Learn the answer to these and other questions at Borders Southgate on Saturday, June 26, when author Patrick Vennebush will talk about his book, Math Jokes 4 Mathy Folks.

Where:
Borders Books, Music, Movies & Cafe
2120 Southgate Road
Colorado Springs, CO 80906
(719) 632-6611

When:
Saturday, June 26
1-3pm

Venn Diagram for April

Are you aware that April is Math Awareness Month?

And did you know that April is also National Humor Month?

Those two things mean that:

• This is a great time for me to present the Math Joke Hour at the 2010 NCTM Annual Meeting in San Diego, April 21‑24. The Math Joke Hour will be held Wednesday, April 21, at 2:30pm in Room 6E of the San Diego Convention Center. Hope to see you there!
• It’s an even better time to buy Math Jokes 4 Mathy Folks! You can purchase it directly from Robert D. Reed Publishers; or, if you’re attending the NCTM Annual Meeting in San Diego,  you can pick up a copy in the bookstore at a 25% discount.