Data, Science & More

Hacking for a future data flood

Astronomy has always been a “big data science”. Astronomy is an observational science: we just have to wait, watch, see and interpret what happens somewhere on the sky. We can’t control it, we can’t plan it, we can just observe in any kind of radiation imaginable and hope that we understand enough of the physics that governs the celestial objects to make sense of it. In recent years, more and more tools that are so very common in the world of data science have also penetrated the field of astrophysics. Where observational astronomy has largely been a hypothesis driven field, data driven “serendipitous” discoveries have become more commonplace in the last decade, and in fact entire surveys and instruments are now designed to be mostly effective through statistics, rather than through technology (even though it is still stat of the art!).

In order to illustrate how astronomy is leading the revolutions in data streams, this infographic was used by the organizers of a hackathon I went to nearing the end of April:
Streams and volumes of data!

The Square Kilometer Array will be a gigantic radio telescope that is going to result in a humongous 160 TB/s rate of data coming out of antennas. This needs to be managed and analysed on the fly somehow. At ASTRON a hackathon was organized to bring together a few dozen people from academia and industry to work on projects that can prepare astronomers for the immense data rates they will face in just a few years.

As usual, and for the better, smaller working groups split up and started working on different projects. Very different projects, in fact. Here, I will focus on the one I have worked on, but by searching for the right hash tag on twitter, I’m sure you can find info on many more of them!


We jumped on two large public data sets on galaxies and AGN (Active Galactic Nuclei: galaxies with a supermassive black hole in the center that is actively growing). One of them was a very large data set with millions of galaxies, but not very many properties of every galaxy (from SDSS), the other, out of which the coolest result (in my own, not very humble opinion) was distilled was from the ZFOURGE survey. In that data set, there are “only” just under 400k galaxies, but there were very many properties known, such as brightnesses through 39 filters, derived properties such as the total mass in stars in them, the rate at which stars were formed, as well as an indicator whether or not the galaxies have an active nucleus, as determined from their properties in X-rays, radio, or infrared.

I decided to try something simple and take the full photometric set of columns, so the brightness of the objects in many many wavelengths as well as a measure of their distance to us into account and do some unsupervised machine learning on that data set. The data set had 45 dimensions, so an obvious first choice was to do some dimensionality reduction on it. I played with PCA and my favorite bit of magic: t-SNE. A dimensionality reduction algorithm like that is supposed to reveal if any substructure in the data is present. In short, it tends to conserve local structure and screw up global structure just enough to give a rather clear representation of any clumping in the original high dimensional data set, in two dimensions (or more, if you want, but two is easiest to visualize). I made this plot without putting in any knowledge about which galaxies are AGN, but colored the AGNs and made them a bit bigger, just to see where they would end up:
t-SNE representation of galaxy data from ZFOURGE

To me, it was absolutely astonishing to see how that simple first try came up with something that seems too good to be true. The AGN cluster within clumps that were identified without any knowledge of the galaxies having an active nucleus or not. Many galaxies in there are not classified as AGN. Is that because they were simply not observed at the right wavelengths? Or are they observed but would their flux be just below detectable levels? Are the few AGN far away from the rest possible mis-classifications? Enough questions to follow up!

On the fly, we needed to solve some pretty nasty problems in order to get to this point, and that’s exactly what makes these projects so much fun to do. In the data set, there were a lot of null values, no observed flux in some filters. This could either mean that the observatory that was supposed to measure that flux didn’t point in the direction of the objects (yet), or that there was no detected flux above the noise. Working with cells that have no number at all or only upper limits on the brightness in some of the features that were fed to the machine learning algorithm is something most ML models are not very good at. We made some simple approximations and informed guesses about what numbers to impute into the data set. Did that have any influence on the results? Likely! Hard to test though… For me, this has sprung a new investigation on how to deal with ML on data with upper or lower limits on some of the features. I might report on that some time in the future!

The hackathon was a huge success. It is a lot of fun to gather with people with a lot of different backgrounds to just sit together for two days and in fact get to useful results, and interesting questions for follow-up. Many of the projects had either some semi-finished product, or leads into interesting further investigation that wouldn’t fit in two days. All the data is available online and all code is uploaded to github. Open science for the win!

SAS in a Pythonista’s workflow

Most who know me will probably know that I’m a major fan of Python in particular and open source software in general. Still, in my day job I use SAS quite a lot. Because I will be at the SAS Global Forum in Denver next week, I thought it would be a good time to write about the place of SAS in my work.

First of all, I can obviously not pay for the massive license fees of software like SAS, which also is my biggest objection to using it. So when I do anything on a freelance basis, it will be purely Python, in almost all assignments. In my day job I arrived, now 4,5 years ago, in a team that even was called the “SAS team”. Their main, and in fact for a long time only, tooling was SAS Base. After fighting for a good two years we do now have a decent Anaconda installation with a Jupyter Hub which makes for much faster data exploration, easier analytics and easier and much prettier visualization (no, SAS Visual Analytics is *not* a visualization tool, it’s a dashboarding tool, enormous difference).

Alright, so I can use Python and R and I still use SAS? Indeed I do. I think every tool is good for something. Part of our team is mainly occupied with ETL (extract, transform, load) work, building a good, stable, clean and well structured data warehouse that others (like me) can use at ease. For such work, SAS has some great tools. Moving data from a data base that is designed to make a particular application work smoothly into a data warehouse, that keeps track of the history of that data base as well is not a trivial task and keeping track of all relevant input data base changes is quite some work. This is done with SAS (by others) and results in a data warehouse stored in SAS data sets that I consequently use for data analysis, machine learning, you name it.

From a data warehouse in SAS tables, I find it easiest to do my data collection, merging, aggregation and all that prerequisite work one always needs for analytics in SAS as well. Most of my projects start with data wrangling in SAS, and when I get to a small number of tables that are ready for analysis, analytics or visualization, I move over to Python (where the fun starts).

Yes, I do have my strong opinions about many SAS tools, about SAS as a company, about how they work and about how the world (e.g. auditors, authorities) look at SAS versus how they look at software from the open source realms. Still, I use a tool that is good at what I need from it, if I have access to it. More and better interfacing between Python, R and SAS is well underway and I hope the integration between the two will only become smoother. Hoping that SAS would vanish from my world is just an utterly unrealistic view of the world.

Why upgrade your OS if you don’t need to?

Two weeks ago I had nothing to do on a Saturday afternoon. Geeky as I am, I thought “let’s upgrade Xubuntu” on my laptop. I was still on the 16.04 Long Term Support version and it is 2018 by now, after all. A month or so later the new LTS version, 18.04, would appear, but why wait? That afternoon I had nothing better to do; when the new version comes out, I probably will be occupied. As usual, I googled around about a version upgrade from 16.04 to 17.10 and was, by every single site I encountered on the topic, strongly advised not to try this. But alas, I’m as stubborn as data scientists get and went ahead anyway.

Ouch. There seemed to be no problem at all. The upgrade was fine, except for some small weird looking error from LibreOffice, which I hardly ever use anyway. Everything worked as it worked before, even with the same looks (then why would you upgrade, right?). I played a game of Go for a bit and then went and let all other software do their updates as well. The usual reboot went blazing fast. I was prompted for my password and then the shit hit the fan: black screen. Nothing. Light from under the keyboard, but that was the only sign of life.

That is when you all of a sudden need a second computer. In all my confidence I had not created a bootable USB stick before the upgrade, and I didn’t have an older one with me either. With a terribly slow and annoying Windows laptop I managed to create one and booted the laptop from that. All fine. Good, so let’s replace the new and corrupted OS with something from this stick. Apparently, when I previously had installed my laptop, there seemed to be no need for a mount point for /efi, and now there was. Annoyingly, this needed to be at the start of the partition table, screwing up pre-existing partitions on that disk. At that point I decided: alright, format the disk, decently partition the disk and a fresh install would make my dear laptop all young and fresh again. There are two SSDs in the machine, and there was a back up of my home on the second disk.

The new install works like a charm, and restoring a back up also calls for some more cleaning up. It all looks tidy again.

I know, not everybody will be a fan of the old-school conky stuff on the desktop, but it just tickles my inner nerd a bit (and I actually do use it to monitor the performance when something heavy is running, I sure like it better than a top screen!). Took a decent part of a day, altogether, so next time I might take the advise of fellow geeks a bit more seriously. On the other hand, after mounting the second disk and finding the backups intact, the short moment of relief is worth it.

Off we go with a clean machine!

Craft beer!


One of my major interests outside the geeky realms is craft beer. I’m an avid sampler, try to go to beer festivals whenever I can, open Untappd almost every day, and I sometimes tweet about beer under a different twitter handle than my own: @BeerTweep.


In Dutch I have written (and hope to wrote again soon!) content for Bierista. You may be too lazy to go look on that site, so here’s a list of the specific things I wrote on there (newest first):

Leaving the field: becoming an extronomer

I wrote the post below roughly 5 years ago now. I quit my academic job in astrophysics, turned to industry and became a data scientist. I might at some point write something about how I think about all of this now.

I have decided to quit astronomy and start a job in the ‘real world’

I give up on a dream. I thoroughly enjoy doing astronomy. I have time left on my current contract and am even fairly confident that after this and another limited number of temporary jobs, I could have gotten a more permanent job in the field at some time, somewhere. So why quit? The uncertainty in a career in astronomy is enormous. If you don’t belong to the very top, and I don’t, you will have to go with the flow and move to wherever the field wants you. Sometimes that will get you to a very nice place in every respect (as we had in Baltimore), sometimes the place to work is very nice, but the place to live less so (like our current situation) and without a doubt even less desirable combinations are possible. Not the biggest deal for a short term postdoc (though too bad if it really doesn’t work out), but where will this long sought-after tenure-track job take you? And when?

Everybody who has done it for a while knows it: living far away from family and good friends is not easy. You can and will build up a new social life (if you even care about a social life, which you should), but those close at heart will be far. Too far, often. Our daughter deserves to grow up among the love of her grandparents and the rest of her family, just as well as they deserve to witness Amy growing up. It is a choice that everyone has to make for him-/herself, but for me a career in astronomy does not outweigh this aspect of life.

Contrary to (too) many academics, I believe that jobs outside of academia can be equally interesting. In many jobs, the everyday activities are even very similar to those of an astronomer. I think I have landed such a job. I will work in data science and business intelligence at a relatively small scale health insurance provider. An example of a project could be to detect fraud in their databases of claims, doctors, hospitals etc. (automatically). In my opinion, that is intellectually as challenging as the questions I am working on in astronomy, with the additional benefit that more people than just a handful of colleagues care about what you do.

Doing astronomy is fun. Like me, many colleagues often describe it as getting paid for your hobby. I can now go back to doing it purely as a hobby. There are several projects I will try to stay involved in to some extent, and I have a couple of very small projects in mind that I still can do. One doesn’t need to be a professional astronomer to do something fun and remotely useful. As an extronomer, you can easily be an amateur astrophysicist as well.

There are many aspects of working in astronomy that I will miss. The friendly atmosphere, collegiality and informality are a bless. I have met many great friends, with whom I hope to stay in contact. On the other hand, I am very much looking forward to my new profession and old environment. Even though I will leave the field professionally, at heart and in my way of thinking I will always be an astronomer.

Fraude detecteren door vernuftige analyse

(This is a blog published on the website of my employer, thought it would make a fair placeholder, sorry about the Dutch).

De zorg is al duur zat. Elke extra euro die moet worden uitgegeven omdat een zorgverlener de regels buigt moet achterhaald, of liever nog voorkomen worden. Probleem is dat “die euro” dan wel gevonden moet worden en dat is geen sinecure. Dit komt door enerzijds het enorme volume aan declaraties dat een zorgverzekeraar krijgt van alle zorgverleners bij elkaar en anderzijds het feit dat fraudeurs slimme dingen bedenken om vooral niet op te vallen. Hoe sporen we bij DSW fraudeurs op?

Op zoek naar outliers

Naast de meer traditionele manieren om fraude op te sporen gebruiken we bij DSW ook analysetechnieken om in de grote bergen data die we hebben patronen te ontdekken die zouden kunnen wijzen op frauduleus of ander onwenselijk gedrag. De tandarts die in een jaar 20 vullingen legt bij een familielid of de psychiater die 40 uur per dag zegt te werken is natuurlijk gauw gevonden, maar de meeste boeven pakken het toch slimmer aan. Precies daarom moeten wij nog slimmer zijn!

Voor het detecteren van rotte appels in een fruitmand vol zorgverleners gaan we er over het algemeen van uit dat de grote meerderheid zich niet misdraagt. Dit betekent dat we kunnen zoeken naar zogenaamde “outliers”, personen of instanties die zich net even anders lijken te gedragen dan de norm. Een complicerende factor is dat wij als zorgverzekeraar altijd maar een deel van het gedrag van een zorgverlener kunnen controleren, namelijk alleen de zorg die gedeclareerd wordt voor die patiënten die toevallig bij ons verzekerd zijn. Bij sommige instanties is dat misschien wel 60% van de populatie of meer, maar bij de meesten toch duidelijk minder, en we weten niet eens hoeveel precies. We hebben dus eerst wat werk te verzetten om analyses te doen op grootheden die niet zo gevoelig zijn voor het marktaandeel van DSW, zoals bijvoorbeeld kosten per verzekerde.


De score opbouwen

Alleen controleren op een getal als de gemiddelde kosten per verzekerde dekt de lading ook niet. Als een kwaadwillende zorgverlener dan kleine bedragen declareert voor een hoop mensen, dan gaat hij zelfs lager eindigen in kosten per verzekerde (terwijl hij in het aantal mensen waarvoor iets declareert juist weer zou opvallen). We kijken daarom naar heel veel van dit soort graadmeters tegelijk. Zo bouwen we een “score” op die aangeeft op hoeveel van dit soort anomaliedetecties (en andere controles) een zorgverlener opvalt, en hoe sterk hij opvalt.

Naast het detecteren van grootheden waarop een zorgverlener opvalt door ander gedrag dan zijn/haar concurrenten, kunnen we ook kijken naar signalen van verzekerden (“ik heb die behandeling helemaal niet gehad”), naar simpele regels als “geamputeerde ledematen kun je niet meer breken” of zelfs naar (zakelijke) verbanden tussen verschillende zorgverleners waardoor lucratieve doch frauduleuze werkwijzen kunnen worden overgebracht van de doorgewinterde oplichter naar de crimineel in spe.

Verder onderzoek

Alle ingrediënten van een totaalscore zijn hard te maken met statistiek, wet- en regelgeving of andere afspraken. Door het tweaken van de opbouw van zo’n totaalscore maken we een lijst waarvan we toch met enige zekerheid wel kunnen zeggen dat er iets loos is. Wanneer dergelijke signalen zijn gegenereerd dan pakken andere afdelingen dit doorgaans op voor verder onderzoek. Hiervoor levert ons datateam uiteraard nog wel data en analyses aan, maar het initiatief voor het uiteindelijk aanpakken ligt dan in de handen van de afdelingen die dichter bij de dagelijkse gang van zaken van een zorgverlener staan.