can I add another thought.
WILD system trades SP very well, intra-day. It can't trade ED very well at all. WILD is designed to profit from volatility. It has traded SP for a long time and is still strong. Real time results are much the same as backtesting results. WILD is simple and has few parameters, degrees of freedom, what evers. Optimisation shows that WILD works over a wide range of parameter values.
is WILD a robust even though it trades only one market?
Continuing the story......There is one catch to the WILD system that the trader is not aware of: if SP volatility drops below X, the system is useless. Last week such an event happened (hypothetical). Now it is 5 years later, 2008. Traders are talking about the swinging days of SP volatility that all ended in mid 2003. Since then SP has been tame and quite.....but it trends really well. Most traders added SP to their Aberration portfolio. WILD was ditched as a system after a 50% DD, this happened by late 2003. Some time in 2008 the trader of WILD reminds thinks about his old system and is happy that he no longer trades it. He reminds himself of the importance of a robust system.
Does a system have to work in all market conditions to be called robust?
can one say of a system "if <some market characteristic> remains high/low, this system is robust"
I have a very good surfboard, one of the best I have ever owned. This board rides so well.... so long as the waves are reasonably small and fast, other than that it is useless. A silly parallel, but a tool is a tool. Take a tool out of the purpose/environment it was designed for and it is no longer a relevant tool, just a bad idea.
Another angle of approach (after thought - I got a little carried away here, sorry folks):
(Creationists, please bear with me). It is some massive number of years ago. You are a monkey. You swing in the tress and eat ripe fruit generally safe from Saber Tooth Tigers and other such nasties. You have a few children monkeys and family life is well. As the years pass your forest, and all the ones around you begin to die as a result of bad weather. Not all is lost for you, but you are concerned for your children. You sadly die (sorry, it was in your sleep as an old content monkey). As your children grow up things get more bleak. By middle age they are in real trouble. No more fresh fruit and they have to search for water. They start to venture onto the plains. More time passes and generations come and go. During this period the ventures into the plains for food and water became more and more frequent. One day, your great, great, great,........, great grandchild is out in the plain with his family and decides not to go back to the trees that night. A Saber Tooth Tiger tries to attack him but he runs so fast he and his family escape. There is no way you could have escaped. Little does your descendent know, he is very different from you. His thumbs are in a different position, he stands nearly upright and that nice hair has gone. More centuries pass. The latest member of your family lives in a cave, has a fire and just noticed how useful a round object is to move heavy things. What has happened? You, as a monkey, were robust, but as the environment changed your descendents were not as robust, but it was not their fault. Luckily nature guided your descendents in an evolutionary path. They adapted.
Whilst you were in the trees, eating bananas and picking knits from your children fur, there was a funny creature swimming around in the ocean. You did not know this creature existed. Your descendent in the cave (the one who invented the wheel) had seen this creature. He didn't think anything of it besides the fact that it looked funny, tasted ok but was hard to catch. Today, your latest descendent (remember, you were the monkey) is sitting in front of a Pentium III flatscreen and have ADSL. It is a cold day he turns on the heater. Later, he turns on the TV and watches the nature channel. He is happy to find a special about his favorite creature, the turtle. This living dinosaur has not changed since the monkey at the top of your family tree was alive. Wow, what a robust animal...... so long as he is in the ocean.
(Side note: when I first read of 'The Turtles' I decided that the better reason for the naming was because they are slow, patient, wise trend followers who will last as long as markets trend. But no, it was some aqua farm in Malaysia).