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PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

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WHEN CAN ACCIDENT YEARS BE REGARDED AS DEVELOPMENT YEARS? 245Note what the cumulative forecast, C, consists of in terms of theincrementals:C =57£ 42=38 = (12 + 26 + 19) £ (<strong>18</strong> + 24)=(12 + 26):Hence, we haveP =57£ 42=38 ¡ 42= (12 + 26 + 19) £ (<strong>18</strong> + 24)=(12 + 26) ¡ (<strong>18</strong> + 24)= (12 + 26 + 19) £ (<strong>18</strong> + 24)=(12 + 26) ¡ (<strong>18</strong> + 24)£ (12 + 26)=(12 + 26)= (12 + 26 + 19 ¡ 12 ¡ 26) £ (<strong>18</strong> + 24)=(12 + 26)=19£ (<strong>18</strong> + 24)=(12 + 26) = B:C=A:Every incremental forecast turns out to work the same way (recallthat you must replace unobserved values in A, B and C bytheir forecasts in this formulation). Consequently, when we interchange(i.e., transpose) accident and development years in theoriginal array and apply the chain ladder, note that the same valueis obtained:Considered in terms of cumulative paid loss, it is not immediatelyclear that the chain ladder incremental prediction, P, willnot change as a result of the transposition. However, if you considerit in the incremental paid loss form, while B and C haveinterchanged, their product is obviously the same. Further, A isunchanged, so the forecast is unchanged. In each case, we haveP =19£ (<strong>18</strong> + 24)=(12 + 26).

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