Training effect

Sounds fairly ok.  I never get my anaerobic figure above that even when running on hills.

Doesn't make sense to me. In an interval session on Tuesday I spent 34% in zone 5 and was 3.3 on anaerobic? 40 % was in zone 4. 3.9 on aerobic

Intervals are generally more beneficial than a flat-out run so that tallies.  Don't be taking statistics too seriously anyway as they are only estimates anyway.  There isn't any way to factor things like terrain and wind in

Unless the rapid elevation of HR is counted, too. Alas, I did not make intervals with my F5+, which I still have, but made my first HIIT with my F6X, and it seemed to me at first glance that not just speed counted, but also my HR, more exactly the change of HR. The terrain was mixed, some sprints happened on flat, some on modest climbs.

Wind is not considered by watches, of course, although my Stryd does. Maybe sometime in the future when a power based running workout will be launched by Garmin then the effect of wind will be implicitly incorporated.

Thanks for info. I understand how intervals may show more anaerobic effect however  other 5k or even a 10k race showed anaerobic effects of 3 or 4 a few months ago and my times are only a little faster 5-10 seconds with no improvement in vo2max so assume I'm not fitter. Just inconsistent but as you say it's just algorithm and probably not very accurate

Maybe not inconsistent, just not fully transparent and too complicated to “reverse engineer” it.

A guidance fyka www.firstbeatanalytics.com/.../