cl.Development(), drop_valuation not behaving as expected

[goduckie]

Perhaps this is related to issue #164 in terms of the indexation (i.e. ldfs starting from 24 rather than 12) as one needs to set drop_valuation date offset by 1 period from (at least my) intuition to achieve desired effect.

Would expect setting drop_valuation=1990 to drop the last diagonal in the following example, but it has no effect. Instead one needs to set drop_valuation=1989 in order to drop the last diagonal.

Will this behavior be effected by changes relating to ldf indexation (as per #164)?

import chainladder as cl2
raa = cl.load_sample('raa')

#all valuation dates included
cl.Development().fit_transform(raa).ldf_
          12-24     24-36     36-48     48-60     60-72     72-84     84-96    96-108   108-120
(All)  2.999359  1.623523  1.270888  1.171675  1.113385  1.041935  1.033264  1.016936  1.009217

cl.Development(drop_valuation='1990').fit_transform(raa).ldf_
          12-24     24-36     36-48     48-60     60-72     72-84     84-96    96-108   108-120
(All)  2.999359  1.623523  1.270888  1.171675  1.113385  1.041935  1.033264  1.016936  1.009217

# last diagonal dropped
cl.Development(drop_valuation='1989').fit_transform(raa)
          12-24     24-36     36-48     48-60     60-72     72-84     84-96    96-108  108-120
(All)  3.213415  1.589016  1.292624  1.163187  1.152502  1.043968  1.037964  1.002902      NaN

[jbogaardt]

I don't believe this is related to #164. When using the dropping features of the Development estimator, we are dropping link_ratio entries of the triangle. It is easier to reason which ones to drop by inspecting the link ratios. The latest diagonal for which this triangle has valid link-ratios is valuation year 1989.

> raa.link_ratio
         12-24     24-36     36-48     48-60     60-72     72-84     84-96    96-108   108-120
1981   1.649840  1.319023  1.082332  1.146887  1.195140  1.112972  1.033261  1.002902  1.009217
1982  40.424528  1.259277  1.976649  1.292143  1.131839  0.993397  1.043431  1.033088       NaN
1983   2.636950  1.542816  1.163483  1.160709  1.185695  1.029216  1.026374       NaN       NaN
1984   2.043324  1.364431  1.348852  1.101524  1.113469  1.037726       NaN       NaN       NaN
1985   8.759158  1.655619  1.399912  1.170779  1.008669       NaN       NaN       NaN       NaN
1986   4.259749  1.815671  1.105367  1.225512       NaN       NaN       NaN       NaN       NaN
1987   7.217235  2.722886  1.124977       NaN       NaN       NaN       NaN       NaN       NaN
1988   5.142117  1.887433       NaN       NaN       NaN       NaN       NaN       NaN       NaN
1989   1.721992       NaN       NaN       NaN       NaN       NaN       NaN       NaN       NaN

[goduckie]

My bad, its even in the title of the section "2.2.2. Omitting link ratios"

Thank you for the clarification, that makes sense.