To get started we need to indicate when the Fiscal Period starts and when it ends, hence the two parameters: [CFY Start] and [CFYTD End]. Then we need to determine which data to include in our calculation. The DATEDIFF and DATEADD functions will come in handy for that.

Let’s assume we want to create a Fiscal Year summary for a measure called [Sales] based on the [Order Date]:

FYTD Sales Current =
IF [Order Date] < [CFY Start] THEN Null
ELSEIF [Order Date] > [CFYTD End] THEN Null
ELSE [Sales]
END

To compare the current fiscal year sales with the same period during the previous year, we need to shift everything back by 12 months using the DATEADD function:

FYTD Sales Previous =
IF [Order Date] < DATEADD(‘month’,-12,[CFY Start]) THEN Null
ELSEIF [Order Date] > DATEADD(‘month’,-12,[CFYTD End]) THEN Null
ELSE [Sales]
END

Note: when using the DATEADD function to add or subtract months, we don’t have to worry about leap days – pretty nifty.

A similar approach works for Month to Date calculations. With the DATEDIFF function we get rid of all the data that doesn’t fall into the current month, then we just add up the days we want to include:

FMTD Sales Curent =
IF DATEDIFF(‘month’,[Order Date],[CFYTD End]) <> 0 THEN NULL
ELSEIF [Order Date] > [CFYTD End] THEN NULL
ELSE [Sales]
END

When comparing to the same month a year ago, we can use the same trick of shifting time as we used for our Year to Date Previous calculation:

FMTD Sales Previous =
IF DATEDIFF(‘month’,[CFYTD End],[Order Date]) <> -12 THEN NULL
ELSEIF [Order Date] > DATEADD(‘month’,-12,[CFYTD End]) THEN NULL
ELSE [Sales]
END

Bonus Calculation:

If you need to calculate the number of days in a year, here’s a way that considers Leap Years:

IF (datepart(‘year’,[Order Date]) % 400) = 0 then 366
ELSEIF (datepart(‘year’,[Order Date]) % 100) = 0 then 365
ELSEIF (datepart(‘year’,[Order Date]) % 4) = 0 then 366
ELSE 365
END

The % sign is Tableau’s syntax for modulo calculations.

Enjoy!

Here’s a workbook with examples:

This statement leaves the unfortunate impression that decisions based on theoretical models are somehow suspect and of little value.  The better, but less soundbite-friendly, question should have been whether the model adequately predicts the risk. To make useful predictions, a model needs to do two things:

• account for all key factors that influence the outcome
• quantify how each factor influences the outcome.

To the frustration of passengers, airlines and businesses around the world, the eruption of the Eyjafjallajokull volcano in Iceland presented several challenges for modelers.  Some of the key factors playing a role here include:

• Ash composition: how damaging is it?
• Extent of the cloud: how high, how far and where?
• Aircraft capability: how much ash exposure can airplanes handle?
• The costs of staying grounded vs. the costs of catastrophic failure

These four bullets of course represent just the tip of the ice berg: each of them summarizes a long list of related factors.  For example, while current data is unavailable, scientists have to rely on past experience when estimating the size and weight of ash particles, their chemical make-up and the density of the ash.  To confound matters, predictions regarding wind speed, direction and turbulence need to be considered as well.

So the real question needs to be: how well does the model represent the current situation? How well do we understand what the actual key factors are?  How well can we measure them?  Are we relying purely on past insight or can we refine our knowledge with data from the current situation?  To what extent can current satellite images, air samples and meteorological measurements improve our ability to predict the risk to life and well-being of people and property?

This interactive map on the BBC we site shows one example of how output from such modeling looks by mapping the extent of the plume over several days along with normal flight routes across the Atlantic.  This post in Business Week sheds some light on our spotty knowledge regarding the real risks of volcanic ash.

The other critical question revolves around acceptable risk.  Risk not only originates from ash clouds. It also comes from “playing it safe.”  Billions of dollars in lost revenues and productivity put the livelihood of hundreds of thousands of people at risk.  Every day we accept the risks of driving our car – so at what point do ash clouds represent a higher risk than driving a car?  Even if we could come up with an exact numeric value for these risks, how does the value of human life fit into these equations?

Therein lies the apparent disconnect between statistical models and real life: intangible values sometimes outweigh what can be measured.  The decisions we make depend on how we actually perceive the risk.  Yet, in order to put our perceptions into perspective, we need to have good numbers to guide us – and getting good numbers requires a good model of reality.

So, before we start talking about law suits, we need to accept that risk is inherent to anything we do.  Blaming people for doing the best they can to balance public safety with economic considerations wastes resources that would be better spent on improving our ability to assess and manage risks.  We send unmanned drones to gather combat intelligence, why not modify them to collect air samples?  Why not fund research to create better models for volcanic plumes? Especially if history should repeat itself and Eyjafjallajokull continues to sputter for the next year or two.

Additional Reading

From Eruptions, a blog dedicated to volcanism:

Airlines lobby to reopen European airspace closed by Eyjafjallajökull
Posted on: April 18, 2010 2:30 PM, by Erik Klemetti

http://scienceblogs.com/eruptions/2010/04/airlines_lobby_to_reopen_europ.php

Eyjafjallajökull flight cancellations: How the right decision is being made to look wrong

Posted on: April 22, 2010 9:40 AM, by Erik Klemetti

http://scienceblogs.com/eruptions/2010/04/eyjafjallajokull_flight_cancel.php

Research Gap Left Airlines Exposed to Volcano’s Blast (Update1)

April 22, 2010, 9:05 AM EDT

http://www.businessweek.com/news/2010-04-22/research-gap-left-airlines-vulnerable-to-volcano-s-fire-and-ice.html

Is driving more dangerous than flying through ash?

Page last updated at 10:51 GMT, Wednesday, 21 April 2010 11:51 UK

http://news.bbc.co.uk/2/hi/uk_news/magazine/8633484.stm

Could aircraft dodge the volcanic ash cloud?

Page last updated at 14:06 GMT, Tuesday, 20 April 2010 15:06 UK

By Stephen Mulvey

BBC News

http://news.bbc.co.uk/2/hi/in_depth/8632583.stm

Recriminations erupt in ash-fueled aviation crisis

AP 4/21/2010

By Arthur Max, Associated Press Writer

http://news.yahoo.com/s/ap/eu_iceland_volcano

How volcanic ash could ground your next flight

By Larry Dignan | Apr 15, 2010

http://www.smartplanet.com/business/blog/smart-takes/how-volcanic-ash-could-ground-your-next-flight/5989/?tag=content;col1

How Volcanic Ash Can Kill An Airplane

Apr 15, 2010 09:00 AM

Ray Wert

Story & pictures of KLM Flight 867

http://jalopnik.com/5517775/how-volcanic-ash-can-kill-an-airplane

Iceland Volcano Vs Mt. St. Helens And Airspace

Pilot’s discussion forum

http://www.airliners.net/aviation-forums/general_aviation/read.main/4783270/

Volcanic Ash Contingency Plan

ICAO (International Civil Aviation Organization)

http://www.paris.icao.int/documents_open/files.php?subcategory_id=63

http://www.paris.icao.int/documents_open/show_file.php?id=274