SleepyHead - Change SpO2 baseline? Too many incorrect desats

[joeb]

I've searched this forum and the web for an unambiguous and calculable definition of "Desaturation Event" and of "SpO2 Baseline". I've failed to find anything that I can understand.
 
Here's a typical statement from a typical website:
(https://www.verywellhealth.com/oxygen-de...ex-3015362)
 
The ODI is the number of times per hour of sleep that your blood oxygen level drops by a certain degree from baseline. Drops in blood oxygen level are called desaturations.
 
Is baseline a constant value for each individual, or is it a series of data-points within the set of any given night's data?  
 
If baseline is a series of data-points, how is the series defined?
 
Does the amount of time that the blood oxygen level remains below the baseline alter the calculation of the ODI?
 
Here's a definition of Desaturation Index from the ApneaBoard wiki:
(https://www.apneaboard.com/wiki/index.php/Sleep_Indices)
 
The Desaturation Index (desat index, ODI) is the average number of times blood oxygen levels drop per hour. To be considered a desaturation event, the blood oxygen level must drop 3% from a baseline. For example, a desaturation occurs when blood oxygen drops from 99% to 96% spO2. ODI is the sum of desaturation events divided by the number of hours of sleep.
 
So, assume that the spO2 rate is recorded as follows for each of following twenty data-points that are four seconds apart:
 
01:98%, 
02:97% 
03:96% 
04:96% 
05:96% 
06:95% 
07:94%
08:95% 
09:93%
10:96% 
11:94%
12:92%
13:95%
14:94%
15:92%
15:91%
15:91%
16:91%
17:91%
18:91%
19:93%
20:90%
 
Which data-points represent baseline(s)?
How many 3% desaturation events are there?  How many 4% desaturations?
 
There's probably a simple explanation that I'm failing to grasp.