I am trying to understand progression charts, read many topics, but did I get it right:
stress chart is just EWMA in KJ over past 42 days and are stress levels (low, high peak) calculated same way (ewma 42days back)?
Does the same apply for strain chart - ewma for past 42 days in KJ included strain levels (low, high, peak for past 42 days average - doesnt look so, more like daily)? If so, how is it with focus curve, seems that it changes more rapidly, or does it show my focus from past 42 days too? They both (stress and strain) have same Y description “Daily accumulated strain in KJ” but values are different.
This really need some more explanation, to stop users from trying to figure out or guessing what it could be and how it might work…
Hi Danny,
You have a lot of great questions!
Correct. The stress chart is an EWMA of daily Strain (un-normalized XSS) using the time constants found in account settings → profile. They have different time constants because of the difference in how Low and High/Peak systems respond to training & de-training.
The strain chart precedes the regular XPMC chart. Strain is normalized into Xert’s Strain Score (XSS) by saying that 100 XSS = 1 hour of unfatigued riding at threshold. Raw Strain values (in kJ) are difficult to compare across time (or athletes), which is why we developed XSS to replace it. The Focus duration is calculated similarly to how Focus for a ride is calculated,by looking at your Low, High, and Peak training loads. If you’ve been doing a lot of base training (relatively high Low TL and relatively low High/Peak TL’s) and then you start adding in HIIT training, you’re going to notice the progression focus is going to drop rather quickly.
There are many complexities in Xert that most athletes using our system dont need to worry about (or frankly even care about). We opted to keep things as simple as possible so that it’s easy for athletes that are new to the Xert system (or new to training with Power in general) to pick it up and start training. These types of topics are beyond the scope of most people who are just looking for a training program, or just training to stay fit, which is why we haven’t supplied an overwhelming amount of documentation on them.
Hope this makes sense! Cheers!
My head hurts 
Well, it would probably be a hell of a project now, to get all that in writing, but it seems like - judging by the number of great questions being posted here in general - it would be a valuable resource for those who do seek more in-depth knowledge.
Thank you Scott for clear and thorough explanation, now i see more what happening behind scene.
One thing that i am not still sure at, is Focus duration period calculation, you write that it can change rather quickly, so it is not calculated from values 42days back and can it be change just from one workout to another - from century ride to sprinter depending on current ride, or is more like 7 days average…?
Its an EWMA of your 3 training loads (low, high, & peak) using the time constants in account settings. You’ll notice that it will jump a lot anytime that you perform a good amount of high/peak XSS (notice in my screenshot below that my Focus duration decreases on days following High Intensity training, indicated by blue/green strain). Notice also that focus decreases when only low intensity strain is added to the progression.
To adequately prepare you for your target event, Xert is trying to get your progression focus to arrive at your selected athlete type.
Totally agree. Over analysis leads to paralysis. 
An understanding of the basics helps, but after that I just ride. Xert works. 
I would agree too, although I do like to try and grasp a bit more than factually would be necessary for me 
However, judging from the number of (technical / in depth) posts here, I would not agree with Scott’s assessment that Xert is (kept) as simple as possible
Y’all are curious! I love it! We’ve been working to add more support (more blogs, FAQ’s, and especially the podcasts) to help disseminate this information. One of the challenges that we face (a good challenge, nonetheless) is that our system is constantly evolving as we pursue new opportunities and develop software breakthroughs (pun intended).
That was too easy