Garbage In, Garbage Out Example from yours truly 
I want to bring this to your attention as a simple user case to illustrate WHY good quality accurate data is crucial.
When chest strap battery ran out of juice 
I knew from previous experience that Garmin optical unit is not accurate when heart rate rises above 120ish. I strapped my watch (because I’m a data Addict and can’t run without a watch) over my long sleeved running top, thinking no way it can read the my heart rate through a thick fabric.
But check this out
Look how quickly hr (red line) increased and stayed elevated. Look at the random
drops and increases. this was an aerobic run with my dog 
look at wr (green dotted line) decrease steady (min around 25%) you can clearly tell when my dog wanted to stop and do his business. WR recovers slightly.
(Sometimes the optical HR monitors actually pick up your stride frequency as HR, and that’s what I think happened here. )
Next, check out my comment
Baffled how optical could measure hr through a sleeve. I went on with my day and forgot to delete the data.
New threshold detection method picked my max heart rate and gave me a new threshold of 188 (my heart would be jumping out of my perimenopausal chest at this rate!) from this data set (since I forgot to delete this garbage).
Key take away message:
If your newly updated threshold HR seems a bit off - make sure to check your data for errors / garbage data points, and delete those.
If you are using optical HR measurements, just note that it is much better to use chest strap.
Have a great weekend,
MJ