Post by Rob Collie

### First, a Few Quick Updates…

**1) Just a reminder that enrollments are open for the live, in-person two-day classes** taught by me (Rob) in **Indianapolis** and **Washington DC** (in October and November, respectively.) Sign up now – those dates will be upon us sooner than we all realize!

**2) The pre-order for DAX 2nd Edition is going Great!** Thanks to everyone who has pre-ordered and/or contributed. **PRE ORDER HERE** if you have not already, and don’t forget the exclusive swag rewards like the sticker, poster, and t-shirt.

**3) Ola Freaking Rosling is now using Power BI, and threw us a “shout-out!”** OMG, check out this tweet. I nearly peed myself:

```
```If I hadn’t heard so much Anti-Microsoft-Propaganda, I would’ve solved my data-integration faster with Power BI & Power Query @powerpivotpro

— Ola Rosling (@OlaRosling) September 16, 2015

**If you don’t recognize the name, his dad is Hans Rosling,** the speaker in the first TED talk I ever watched:

**Click Image to View the Talk**

**…In which he did the animated bubble chart demo that I’m 100% positive** was the inspiration for Amir Netz adding animated bubble charts in the next two product releases

**Here’s Ola and Hans co-presenting on the state of world health** and social justice, via data of course:

**Again, Click to View the Talk**

**A celebrity in that Google-y, Apple-y, Silicon Valley-y** “doing good with data in the public interest” space using Power BI and saying that Power BI is awesome, and that he’d previously been scared off by all the anti-MS propaganda… well that is simply awesome.

**But I won’t lie – that he gave US a mention in that tweet** – I *do* find that to be even cooler. I’m human. I can’t help it. I smile every time I think about it.

### Slaying the “White Whale” of Variable-Rate Forecasting with PRODUCTX

**“Arrrrr! We Be Meeting Again!”**

**OK OK… time to circle back to one of my only “defeats” EVER,** and settle the score! Yes, this problem… well it beat me up pretty badly about 18 months ago. I eventually solved it, but not in a way where the formula was even remotely explainable or maintainable.

**I’ve done some ****posts**** about exponential growth** forecasting **before**. In both of those posts, I used the POWER() function to multiply out the increasing or decreasing series. (10 percent growth each year, for five years translates to 1.10 raised to the 5th power).

**But both of those posts assume the growth factor is STATIC.** Meaning, if you forecast 10 percent growth per period, it’s 10 percent growth for EVERY period. That nice, steady percentage allows us to use the POWER function to handle all the multiplication.

**Those were easy mode, in other words.** But then, later, I wrote **another post** that handled *variable* rates. I leaned heavily on Jeffrey Wang from Microsoft for that one, because DAX didn’t have a PRODUCTX function at the time. That was some complex mathmagic-land stuff – converting a multiplication series into a sum by using logarithms and exponents, but it got the job done.

**This was harder: Each year had its own specified growth rate.
**

*(But STILL… that rate applied to EVERYONE for that year – my next example is worse)*

**Still, though, even THAT was easy mode compared to** the problem I encountered a year later!

**What if your growth rates are still variable, but you have multiple populations** of customers moving through the forecast, and as those populations “age,” we must look up their growth rate based on how long ago we acquired them? Meaning, for a given calendar year’s forecast, each group of customers (grouped by their “age”) has to be treated a bit differently?

**Here, it’s probably easier to show you:**

**The Left Hand Column is No Longer a Calendar Year – It’s a Customer’s “Age!”
**

*(Oh, what’s that you say? That table looks funny? It’s Power Pivot in Excel 2016!)*

**In short, THIS problem was a farkin disaster.** The crazy “logarithms, exponents, and SUMX” workaround to create our own version of PRODUCTX was hard enough on its own, but when I THEN had to “nest” THAT contraption of a formula inside ANOTHER crazy series of loops, well, it would have given Obi-Wan one of those terrible disturbances in the Force. It was an unsatisfactory result.

### Now We Have PRODUCTX, for Realz!

**With Excel 2016 and Power BI Desktop, though, we now DO have a real PRODUCTX function,** so that “inner” complexity is removed. At least in theory. So I’ve been meaning to circle back around and give it a shot.

**In short… I beat this sucker.** It was still a bit chewy, but seriously, 20x easier than the last time I tried it (without benefit of PRODUCTX).

Here’s the model:

**Three Tables, No Relationships.**

**The Three Tables: Intentionally Small, But this Technique Will Work With “Bigger” Data**

### Description of the Problem

**Let’s describe the problem in English, **using the tables above as a reference:

**Each year from 2008 through 2015, we have a “known” number**of new customers who “walked in the door.”**We also have research that tells us that one year after we acquire a customer,**there’s a 70% chance that customer is still around.**Any customer who “survives” that first year**then has a 60% chance of sticking around for the following year.**We actually get a “bump” in customer loyalty in year three**– if you’ve stuck with us that long, chances are good that you REALLY like us, and we’ve found we retain 80% of such customers for the next year. (Of course, that’s 80% of 60% of 70% of the original incoming customers, so it’s still a minority in the bigger picture).**But then, customers tend to become “satisfied” or “weary.”**For whatever reason, customer retention falls off after that, and in year 4 we only keep 50%, followed by 30%, 20%, 10%, and ultimately 0%.**So, if we want to project how many customers we’re going to have**“in the system” for year 2019, we have a very complex problem. Each of the 2008-2015 populations must be “aged” differently, and then all of those results must be blended together into a unified projection. Yikes.

### Diagram of the Problem

If we want to forecast total customers for 2016, it would look like this:

**If We Add X + Y + Z, We Get Our Answer
**

*(There are actually other populations involved, too, Like 2012 and 2011, But The Diagram Was Getting Too Big. So it would actually be more like U + V + W + X + Y + Z, but you get the idea.)*

### OK, Enough Setup. Here’s the Resulting Pivot.

**See? Done. That Last Column is the Final Measure.**

### The Formulas

From right to left in the pivot above:

Total Projected Customers:=

[Continuing Customers]+[Customers Acquired]

Customers Acquired:=

CALCULATE(SUM(Incoming[New Customers]),

FILTER(Incoming, Incoming[Year]=MAX(Years[Year])))

Continuing Customers:=

SUMX(FILTER(Incoming, Incoming[Year]<MAX(Years[Year])),

PRODUCTX(FILTER(Retention,

Retention[Year After Being Acquired] <=

MAX(Years[Year])-Incoming[Year]

),

Retention[Retention Rate]

)

*Incoming[New Customers])

**OK, so that third formula was the hard one.** But WAY easier than before, when we didn’t have PRODUCTX!

**Oh??? You want me to EXPLAIN it??? Sheesh folks, it’s 1 am here **and I’m catching a taxi at 6:30 am. I’m gonna be a zombie tomorrow as I fly from one side of the country to the other.

**Seriously, I’m just gonna have to take a rain check for tonight. ** Maybe I’ll explain, tomorrow, in the comments