## Archive for December 23, 2014

### Hold On… *How Many* Copies?

How many copies of *Math Jokes 4 Mathy Folks* do you think sold last week?

Make Your Prediction Here (Google Form) |

Why would you want to make a prediction? Well, lots of reasons…

- Like the author (and readers) of this blog, you’re a math geek.
- You swoon at the sight of data.
- You’ve never met a puzzle you didn’t like.
- You want to show the world how awesome you are.
**You’d like to win a signed copy of***Math Jokes 4 Mathy Folks*, some cool MJ4MF stickers, and a surprise gift, all shipped to you in exclusive MJ4MF packaging!- All of the above.

If you’re reading this blog, then you surely love being alive in the Age of Big Data. I love it, too, and I devour any data that I can get my hands on.

Amazon feeds my desire by providing two valuable pieces of data about * Math Jokes 4 Mathy Folks*. First, they provide the

**sales rank**for the book, which is updated hourly. Second, they provide

**weekly sales data**. The downside to this latter stat is the delay in its release — they provide data for Monday-Sunday, but it isn’t released until the following Friday. The upside is that big dorks like me use the time from Monday through Thursday to make predictions.

The chart below shows the average sales rank and weekly sales for Nov 24 through Dec 14. (The “average sales rank” is the average of the sales ranks for the seven days each week. Although it’s updated hourly, I don’t have the time to check it that frequently, so I rely on Author Central, which reports the sales rank at the end of each day.) It also shows the average sales rank (but not sales) for last week, Dec 15‑21.

Week |
Amazon Sales Rank(Weekly Average) |
WeeklySales |

Nov 24-30 |
4,742 | 114 |

Dec 1-7 |
3,437 | 279 |

Dec 8-14 |
2,390 | 435 |

Dec 15-21 |
2,063 | ? |

**The question: How many copies of MJ4MF were sold last week?**

Oh, sure… I could just wait until Friday to find out — but what fun would that be?

Instead, I constructed several mathematical models, and then I tweaked them to predict how many books were sold. The tweaks were based on some things I’ve learned over the past couple of years:

- Holiday sales are most vigorous in the first two weeks of December. They slow down a bit in the third week. Consequently, a sales rank of 1,655 on Dec 1 does not equal a sales rank of 1,655 on Dec 21.
- The long-term trend is not linear. In fact, this graph from Foner Books shows that it’s logarithmic.

Which brings us to the contest. Go to the Google form and **enter your prediction and email address**. (The email is only so I can contact you if you win.) **Closest guess to the actual number of sales will win the grand prize.** In the event of a tie, a winner will be randomly selected (or if I’m feeling generous, maybe there will be multiple winners… it’s hard to predict my disposition on any given day).

So, what are you waiting for? Open Excel or SPSS or your stat software of choice, muddle through a few regressions, and submit your entry!

**Winners will be announced on Saturday, December 27, 2014.** The exact time will depend on what time I roll out of bed, what activities my wife and kids propose for the day, and my particular disposition on Saturday. On second thought… safest if you check back on Sunday.

Good luck!