Multi-step trending behaving unexpectedly - more details on date entry needed

[vdunaway]

Could someone please provide more details on how the multipart trending works, specifically the math behind what chainladder is doing and how to properly enter the dates for what you intend? I've been toggling the start/end dates and it's not working as intended, thus, potentially the documentation/user-guide could be enhanced regarding this.

For example, say I want to apply a multi-step trending that incorporates the following:

Trends for Year X-1 to X*
	Annual
Year	Trend Rate
Prior to 2021	3.5%
2021	10.0%
2023	5.0%
Trend To Date =	12/31/2024
*trend rate remains the same until
new trend in list

When I run through chainladder, I don't get what I was expecting. Am I inputting the dates incorrectly?

p = pd.DataFrame({'Year': list(range(2014,2024)), 'Premium': [10000]*10})
p_tri = cl.Triangle(p, origin='Year', columns='Premium', cumulative = True)

test = cl.Trend(
    trends=[.05, .1, .035],
    dates=[('2024-12-31', '2022'),('2022', '2020'),('2020','1990')]
).fit(p_tri).trend_

test
Out[71]: 
          2023
2014  1.661630
2015  1.605439
2016  1.551149
2017  1.498695
2018  1.448014
2019  1.399048
2020  1.278334
2021  1.162121
2022  1.102500
2023  1.050000

Dissecting the chainladder output and comparing to expectation, how would one enter the dates into cl.Trend() to obtain the expected result?

	Chainladder Output	Chainladder Resulting YoY Trend Factor	Expected YoY Trend Factor	Expected Result
2014	1.6616298	1.0350000	1.0350000	1.6398573
2015	1.6054394	1.0350000	1.0350000	1.5844032
2016	1.5511492	1.0350000	1.0350000	1.5308244
2017	1.4986949	1.0350000	1.0350000	1.4790574
2018	1.4480144	1.0350000	1.0350000	1.4290409
2019	1.3990477	1.0944308	1.0350000	1.3807159
2020	1.2783336	1.1000000	1.1000000	1.3340250
2021	1.1621214	1.0540784	1.1000000	1.2127500
2022	1.1025000	1.0500000	1.0500000	1.1025000
2023	1.0500000	1.0500000	1.0500000	1.0500000

[kennethshsu]

Hi @vdunaway,

I talked to @jbogaardt, and we agreed that the implementation and documentation don't really make sense in its current stage. We agreed that a new function should be implemented, which will likely be named chainladder.TrendConstant(). This way the naming convention matches other similar functions like chainladder.DevelopmentConstant().

I'll try to work on this in the near future. But meanwhile, if you want to help with test cases, please feel free to post them here.