Add an option to choose the calculation method (Knaus-Ogino, Standard Days)

[arnowelzel]

Some people prefer the use of "standard days" for calculation as an alternative to Knaus-Ogino. Standard days is only possible if the cycle length is between 26 and 32 days.

Using an option in the settings it will be possible to switch between the methods. The default will be "Knaus-Ogino" but one can change to "standard days" as well.

Technically this includes:

• a new preference to choose the calculation method
• extending the existing calculation to handle a specific method
• an additional check for the cycle lengths to display a warning if "standard days" is used and the cycles are shorter than 26 days or longer than 32 days

[TPS]

@arnowelzel Also, perhaps a way to tell how much the calculation methods deviate from each other, given the current data set, so that 1 can make an informed decision as to which method fits 1self better?

Especially, why not add 1st-class support for basal-temperature method (recommended all over app literature/docs as superior), & then allow switching/comparison between all these?

What I mean by 1st-class support? Right now, all 1 can do is "☑️ Temperature fluctuations" & co-opt Notes field. Better would be to have dedicated temperature field (in 1's choice of K/°C/°F/whatever, as precise as 0.01°, if wanted), which would enable graphics/prediction/&c, as other method(s) do.

[arnowelzel]

@TPS

The "Temperature fluctuations" is only there as a symptom - for example when having flushes which may indicate the beginning of the menopause. And I asked a couple of women which symptoms should be in the list - this was one of them.

I did not implement the temperature method yet, because it can give a false sense of confidence. There are already reports from women who trusted a birth control app based on the temperature method to avoid pregnancy - and got pregnant:

https://www.theguardian.com/society/2018/jul/21/colossally-naive-backlash-birth-control-app

Cite:

"In January, a major Swedish hospital reported that 37 of the 668 women who had sought an abortion there between September and December 2017 were using Natural Cycles as their sole birth control, and the Medical Products Agency of Sweden began to investigate."

I hope you understand, that I don't want to take the responsibility for unwanted pregnancies by offering a method which claims to be more "precise" but heavily depends on how people use the app.

[TPS]

I just noticed your extensive reply — I apologize for my inattention.

Your reasoning is precisely why I asked for this to part of the set of deviations. That way, folks should realize that, since the methods don't agree, that, @ best, the indicators should be taken w/ a grain lot of salt.

[hex-m]

Looking at the effectiveness of the different calendar/temperature/cervical mucus-based techniques it seems the "symptothermal method" is the most reliable one.

I hope you understand, that I don't want to take the responsibility for unwanted pregnancies by offering a method which claims to be more "precise" but heavily depends on how people use the app.

I don't think anyone has the right to blame you for unwanted pregnancies. You also clearly state the "Limitation of Liability" in the software license. And there are others (german) who think we need Free Software solutions to track such intimate details.

In any case you just implement the different algorithms and having more algorithms to choose from would hopefully just mean that people do their own research on the effectiveness of the different methods.

Thank you for your work and for this wonderful piece of software! <3

[mgth]

In a humanitarian project, we would like to use Periodical, but the Founder prefer Standard Days method, I think it would be nice to have a different color for the days where the two methods are OK, and when they differ. The user would not have to choose.

[arnowelzel]

After reviewing the change I in #91 think it is too confusing to have two different colors for "fertility". I will change this to an option in the preferences - so one can choose between Knaus-Ogino and Standard Days as originally proposed.

Furthermore Standard Days are only possible if the cycle is between 26 and 32 days. If a cycle is shorter longer, the calculation is not useful - depending on what you want to achieve with it. For shorter or longer cycles the fertile period maybe longer than expected, start earlier than expected or and later than expected. In this case there should be some warning in the affected cycle that the calculated values may not be correct due to its length.

[zikalify]

In the Help page on the app it says standard days fertile days are identified with a green tile, not sure if you are planning to remove this.

If it's easy enough, the app could look at past cycles and automatically determine if SDM or KO methods should be used. You can already see the shortest and longest cycles so it could just check these and if the longest and shortest are between 26-32 days then use green tiles (or keep blue ones for simplicity) and calculate based on the SDM then for people who fall outside of this go back to KO method.

Either way, good app! Personally, I am happy with how it works already.

[arnowelzel]

Sorry, this slipped through, eventhough the selection of the calculation method is not available yet in the UI.

Having different methods combined seems not to be a good choice since the calculation method of the calendar display should be consistent and not depending on the cycle length. Also the colors have a meaning based on the scheme of Maria Hengstberger (also see https://de.wikipedia.org/wiki/Maria_Hengstberger):

Red - menstruation started
Yellow - infertile like the dry sand in the desert
Blue - like the water which makes the desert fertile again

I am still a bit reluctant to use green instead of blue because of that specific meaning of the colors. I'll see how I can solve this in a better way instead of just using different colors.

[TPS]

I'll see how I can solve this in a better way instead of just using different colors.

@arnowelzel This is a good idea also for a11y reasons. Have you run Periodical through any such tool, such as Kontrast (A Color Contrast Checker) or Google's?