Percent loss covered on average with a deductible

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Suppose you have auto insurance with a deductible of $200, and with no restriction on maximal payment. The probability of a loss is 0.10, and suppose that the distribution of the loss is exponential with a mean of $1000.



What percentage of the loss does the insurance cover on average? The answer in the back of the book is 57.4%, but I am not sure how to reproduce it. This is how I thought of doing the problem:



Say that $1000 = 1 unit of money (for ease of calculation), so that the deductible $d = 0.20$



The payment function may be described as $$r(X) = begincases 0, quad quad x leq 0.20 \ x-0.2, quad x > 0.20 endcases $$



where $x$ is the realization of the stochastic variable $$X = begincases xi, quad p = 0.10 \ 0, quad p = 0.90 endcases $$



Here, $xi$ is the exponentially distributed loss. Since we are taking $1000 to be a single unit of money, this just has a mean of 1.



Since we are interested in the percentage of the loss covered, we are assuming that a loss has occurred and that that loss is larger than the deductible. So, the expected value of the loss under this condition is
$$E[xi | xi > 0.20] = 0.2 + E[xi] = 0.2 + 1 = 1.2$$
which is just a result of the shifted exponential distribution. In this case, the insurance will cover 1 unit of money, or $1000, and so we have 83% coverage on average when there is a loss. (1 divided by 1.2).



What is the mistake?







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    up vote
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    Suppose you have auto insurance with a deductible of $200, and with no restriction on maximal payment. The probability of a loss is 0.10, and suppose that the distribution of the loss is exponential with a mean of $1000.



    What percentage of the loss does the insurance cover on average? The answer in the back of the book is 57.4%, but I am not sure how to reproduce it. This is how I thought of doing the problem:



    Say that $1000 = 1 unit of money (for ease of calculation), so that the deductible $d = 0.20$



    The payment function may be described as $$r(X) = begincases 0, quad quad x leq 0.20 \ x-0.2, quad x > 0.20 endcases $$



    where $x$ is the realization of the stochastic variable $$X = begincases xi, quad p = 0.10 \ 0, quad p = 0.90 endcases $$



    Here, $xi$ is the exponentially distributed loss. Since we are taking $1000 to be a single unit of money, this just has a mean of 1.



    Since we are interested in the percentage of the loss covered, we are assuming that a loss has occurred and that that loss is larger than the deductible. So, the expected value of the loss under this condition is
    $$E[xi | xi > 0.20] = 0.2 + E[xi] = 0.2 + 1 = 1.2$$
    which is just a result of the shifted exponential distribution. In this case, the insurance will cover 1 unit of money, or $1000, and so we have 83% coverage on average when there is a loss. (1 divided by 1.2).



    What is the mistake?







    share|cite|improve this question





















      up vote
      0
      down vote

      favorite









      up vote
      0
      down vote

      favorite











      Suppose you have auto insurance with a deductible of $200, and with no restriction on maximal payment. The probability of a loss is 0.10, and suppose that the distribution of the loss is exponential with a mean of $1000.



      What percentage of the loss does the insurance cover on average? The answer in the back of the book is 57.4%, but I am not sure how to reproduce it. This is how I thought of doing the problem:



      Say that $1000 = 1 unit of money (for ease of calculation), so that the deductible $d = 0.20$



      The payment function may be described as $$r(X) = begincases 0, quad quad x leq 0.20 \ x-0.2, quad x > 0.20 endcases $$



      where $x$ is the realization of the stochastic variable $$X = begincases xi, quad p = 0.10 \ 0, quad p = 0.90 endcases $$



      Here, $xi$ is the exponentially distributed loss. Since we are taking $1000 to be a single unit of money, this just has a mean of 1.



      Since we are interested in the percentage of the loss covered, we are assuming that a loss has occurred and that that loss is larger than the deductible. So, the expected value of the loss under this condition is
      $$E[xi | xi > 0.20] = 0.2 + E[xi] = 0.2 + 1 = 1.2$$
      which is just a result of the shifted exponential distribution. In this case, the insurance will cover 1 unit of money, or $1000, and so we have 83% coverage on average when there is a loss. (1 divided by 1.2).



      What is the mistake?







      share|cite|improve this question











      Suppose you have auto insurance with a deductible of $200, and with no restriction on maximal payment. The probability of a loss is 0.10, and suppose that the distribution of the loss is exponential with a mean of $1000.



      What percentage of the loss does the insurance cover on average? The answer in the back of the book is 57.4%, but I am not sure how to reproduce it. This is how I thought of doing the problem:



      Say that $1000 = 1 unit of money (for ease of calculation), so that the deductible $d = 0.20$



      The payment function may be described as $$r(X) = begincases 0, quad quad x leq 0.20 \ x-0.2, quad x > 0.20 endcases $$



      where $x$ is the realization of the stochastic variable $$X = begincases xi, quad p = 0.10 \ 0, quad p = 0.90 endcases $$



      Here, $xi$ is the exponentially distributed loss. Since we are taking $1000 to be a single unit of money, this just has a mean of 1.



      Since we are interested in the percentage of the loss covered, we are assuming that a loss has occurred and that that loss is larger than the deductible. So, the expected value of the loss under this condition is
      $$E[xi | xi > 0.20] = 0.2 + E[xi] = 0.2 + 1 = 1.2$$
      which is just a result of the shifted exponential distribution. In this case, the insurance will cover 1 unit of money, or $1000, and so we have 83% coverage on average when there is a loss. (1 divided by 1.2).



      What is the mistake?









      share|cite|improve this question










      share|cite|improve this question




      share|cite|improve this question









      asked Jul 20 at 6:45









      Marcel

      1123




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          2 Answers
          2






          active

          oldest

          votes

















          up vote
          0
          down vote



          accepted










          The error comes from the statement in bold:




          Since we are interested in the percentage of the loss covered, we are
          assuming that a loss has occurred and that that loss is larger than
          the deductible.




          This is a mistaken interpretation of the question. Instead, the question is implying that when a loss happens, some of these losses are below the deductible, and therefore not covered. In such a case, the percentage of loss covered is $0%$.



          Consequently, the calculation of the expected proportion of loss covered should be $$operatornameEleft[frac(X - 0.2)_+Xright],$$ where $(X - 0.2)_+ = max(0, X - 0.2)$ is the claim payment and $$X sim operatornameExponential(1)$$ is the ground-up loss given that a loss occurs. Note we do not need $xi$ nor do we need the information that a loss occurs with probability $0.1$, because the first part of your statement "we are assuming a loss has occurred" is in fact correct.



          Then the calculation proceeds as
          $$beginalign*
          operatornameEleft[frac(X - 0.2)_+Xright]
          &= operatornameE[0 mid X le 0.2]Pr[X le 0.2] + operatornameEleft[1 - frac15X ,bigg| ; X > 0.2 right]Pr[X > 0.2] \
          &= e^-1/5 - int_x=0.2^infty frac15x e^-x , dx \
          &approx 0.574201.
          endalign*$$






          share|cite|improve this answer




























            up vote
            0
            down vote













            I don't see a basis in the problem statement for conditioning on $xigt0.20$. Quite to the contrary, it seems to me that if you want to know the average percentage of the loss covered by your insurance, the fact that it covers $0%$ if the loss is below the deductible seems like a very relevant piece of information to include in the calculation.



            Without the conditioning, you get



            $$E[r(X)]=mathrm e^-0.2cdot1=mathrm e^-0.2approx81.9%;,$$



            slightly less than your value under conditioning. This, however, is not the average percentage of loss covered; it's the average loss covered, expressed as a percentage of the average loss incurred. The average percentage of loss covered is



            $$
            left(1-mathrm e^-0.2right)cdot0+mathrm e^-0.2int_0^inftyfrac xx+0.2,mathrm e^-xmathrm dx=0.2operatornameEi(-0.2)+mathrm e^-0.2approx57.4%;,
            $$



            where $operatornameEi$ denotes the exponential integral.






            share|cite|improve this answer





















            • Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
              – Marcel
              Jul 20 at 7:21










            • @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
              – joriki
              Jul 20 at 7:28











            Your Answer




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            2 Answers
            2






            active

            oldest

            votes








            2 Answers
            2






            active

            oldest

            votes









            active

            oldest

            votes






            active

            oldest

            votes








            up vote
            0
            down vote



            accepted










            The error comes from the statement in bold:




            Since we are interested in the percentage of the loss covered, we are
            assuming that a loss has occurred and that that loss is larger than
            the deductible.




            This is a mistaken interpretation of the question. Instead, the question is implying that when a loss happens, some of these losses are below the deductible, and therefore not covered. In such a case, the percentage of loss covered is $0%$.



            Consequently, the calculation of the expected proportion of loss covered should be $$operatornameEleft[frac(X - 0.2)_+Xright],$$ where $(X - 0.2)_+ = max(0, X - 0.2)$ is the claim payment and $$X sim operatornameExponential(1)$$ is the ground-up loss given that a loss occurs. Note we do not need $xi$ nor do we need the information that a loss occurs with probability $0.1$, because the first part of your statement "we are assuming a loss has occurred" is in fact correct.



            Then the calculation proceeds as
            $$beginalign*
            operatornameEleft[frac(X - 0.2)_+Xright]
            &= operatornameE[0 mid X le 0.2]Pr[X le 0.2] + operatornameEleft[1 - frac15X ,bigg| ; X > 0.2 right]Pr[X > 0.2] \
            &= e^-1/5 - int_x=0.2^infty frac15x e^-x , dx \
            &approx 0.574201.
            endalign*$$






            share|cite|improve this answer

























              up vote
              0
              down vote



              accepted










              The error comes from the statement in bold:




              Since we are interested in the percentage of the loss covered, we are
              assuming that a loss has occurred and that that loss is larger than
              the deductible.




              This is a mistaken interpretation of the question. Instead, the question is implying that when a loss happens, some of these losses are below the deductible, and therefore not covered. In such a case, the percentage of loss covered is $0%$.



              Consequently, the calculation of the expected proportion of loss covered should be $$operatornameEleft[frac(X - 0.2)_+Xright],$$ where $(X - 0.2)_+ = max(0, X - 0.2)$ is the claim payment and $$X sim operatornameExponential(1)$$ is the ground-up loss given that a loss occurs. Note we do not need $xi$ nor do we need the information that a loss occurs with probability $0.1$, because the first part of your statement "we are assuming a loss has occurred" is in fact correct.



              Then the calculation proceeds as
              $$beginalign*
              operatornameEleft[frac(X - 0.2)_+Xright]
              &= operatornameE[0 mid X le 0.2]Pr[X le 0.2] + operatornameEleft[1 - frac15X ,bigg| ; X > 0.2 right]Pr[X > 0.2] \
              &= e^-1/5 - int_x=0.2^infty frac15x e^-x , dx \
              &approx 0.574201.
              endalign*$$






              share|cite|improve this answer























                up vote
                0
                down vote



                accepted







                up vote
                0
                down vote



                accepted






                The error comes from the statement in bold:




                Since we are interested in the percentage of the loss covered, we are
                assuming that a loss has occurred and that that loss is larger than
                the deductible.




                This is a mistaken interpretation of the question. Instead, the question is implying that when a loss happens, some of these losses are below the deductible, and therefore not covered. In such a case, the percentage of loss covered is $0%$.



                Consequently, the calculation of the expected proportion of loss covered should be $$operatornameEleft[frac(X - 0.2)_+Xright],$$ where $(X - 0.2)_+ = max(0, X - 0.2)$ is the claim payment and $$X sim operatornameExponential(1)$$ is the ground-up loss given that a loss occurs. Note we do not need $xi$ nor do we need the information that a loss occurs with probability $0.1$, because the first part of your statement "we are assuming a loss has occurred" is in fact correct.



                Then the calculation proceeds as
                $$beginalign*
                operatornameEleft[frac(X - 0.2)_+Xright]
                &= operatornameE[0 mid X le 0.2]Pr[X le 0.2] + operatornameEleft[1 - frac15X ,bigg| ; X > 0.2 right]Pr[X > 0.2] \
                &= e^-1/5 - int_x=0.2^infty frac15x e^-x , dx \
                &approx 0.574201.
                endalign*$$






                share|cite|improve this answer













                The error comes from the statement in bold:




                Since we are interested in the percentage of the loss covered, we are
                assuming that a loss has occurred and that that loss is larger than
                the deductible.




                This is a mistaken interpretation of the question. Instead, the question is implying that when a loss happens, some of these losses are below the deductible, and therefore not covered. In such a case, the percentage of loss covered is $0%$.



                Consequently, the calculation of the expected proportion of loss covered should be $$operatornameEleft[frac(X - 0.2)_+Xright],$$ where $(X - 0.2)_+ = max(0, X - 0.2)$ is the claim payment and $$X sim operatornameExponential(1)$$ is the ground-up loss given that a loss occurs. Note we do not need $xi$ nor do we need the information that a loss occurs with probability $0.1$, because the first part of your statement "we are assuming a loss has occurred" is in fact correct.



                Then the calculation proceeds as
                $$beginalign*
                operatornameEleft[frac(X - 0.2)_+Xright]
                &= operatornameE[0 mid X le 0.2]Pr[X le 0.2] + operatornameEleft[1 - frac15X ,bigg| ; X > 0.2 right]Pr[X > 0.2] \
                &= e^-1/5 - int_x=0.2^infty frac15x e^-x , dx \
                &approx 0.574201.
                endalign*$$







                share|cite|improve this answer













                share|cite|improve this answer



                share|cite|improve this answer











                answered Jul 20 at 7:12









                heropup

                59.7k65895




                59.7k65895




















                    up vote
                    0
                    down vote













                    I don't see a basis in the problem statement for conditioning on $xigt0.20$. Quite to the contrary, it seems to me that if you want to know the average percentage of the loss covered by your insurance, the fact that it covers $0%$ if the loss is below the deductible seems like a very relevant piece of information to include in the calculation.



                    Without the conditioning, you get



                    $$E[r(X)]=mathrm e^-0.2cdot1=mathrm e^-0.2approx81.9%;,$$



                    slightly less than your value under conditioning. This, however, is not the average percentage of loss covered; it's the average loss covered, expressed as a percentage of the average loss incurred. The average percentage of loss covered is



                    $$
                    left(1-mathrm e^-0.2right)cdot0+mathrm e^-0.2int_0^inftyfrac xx+0.2,mathrm e^-xmathrm dx=0.2operatornameEi(-0.2)+mathrm e^-0.2approx57.4%;,
                    $$



                    where $operatornameEi$ denotes the exponential integral.






                    share|cite|improve this answer





















                    • Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                      – Marcel
                      Jul 20 at 7:21










                    • @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                      – joriki
                      Jul 20 at 7:28















                    up vote
                    0
                    down vote













                    I don't see a basis in the problem statement for conditioning on $xigt0.20$. Quite to the contrary, it seems to me that if you want to know the average percentage of the loss covered by your insurance, the fact that it covers $0%$ if the loss is below the deductible seems like a very relevant piece of information to include in the calculation.



                    Without the conditioning, you get



                    $$E[r(X)]=mathrm e^-0.2cdot1=mathrm e^-0.2approx81.9%;,$$



                    slightly less than your value under conditioning. This, however, is not the average percentage of loss covered; it's the average loss covered, expressed as a percentage of the average loss incurred. The average percentage of loss covered is



                    $$
                    left(1-mathrm e^-0.2right)cdot0+mathrm e^-0.2int_0^inftyfrac xx+0.2,mathrm e^-xmathrm dx=0.2operatornameEi(-0.2)+mathrm e^-0.2approx57.4%;,
                    $$



                    where $operatornameEi$ denotes the exponential integral.






                    share|cite|improve this answer





















                    • Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                      – Marcel
                      Jul 20 at 7:21










                    • @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                      – joriki
                      Jul 20 at 7:28













                    up vote
                    0
                    down vote










                    up vote
                    0
                    down vote









                    I don't see a basis in the problem statement for conditioning on $xigt0.20$. Quite to the contrary, it seems to me that if you want to know the average percentage of the loss covered by your insurance, the fact that it covers $0%$ if the loss is below the deductible seems like a very relevant piece of information to include in the calculation.



                    Without the conditioning, you get



                    $$E[r(X)]=mathrm e^-0.2cdot1=mathrm e^-0.2approx81.9%;,$$



                    slightly less than your value under conditioning. This, however, is not the average percentage of loss covered; it's the average loss covered, expressed as a percentage of the average loss incurred. The average percentage of loss covered is



                    $$
                    left(1-mathrm e^-0.2right)cdot0+mathrm e^-0.2int_0^inftyfrac xx+0.2,mathrm e^-xmathrm dx=0.2operatornameEi(-0.2)+mathrm e^-0.2approx57.4%;,
                    $$



                    where $operatornameEi$ denotes the exponential integral.






                    share|cite|improve this answer













                    I don't see a basis in the problem statement for conditioning on $xigt0.20$. Quite to the contrary, it seems to me that if you want to know the average percentage of the loss covered by your insurance, the fact that it covers $0%$ if the loss is below the deductible seems like a very relevant piece of information to include in the calculation.



                    Without the conditioning, you get



                    $$E[r(X)]=mathrm e^-0.2cdot1=mathrm e^-0.2approx81.9%;,$$



                    slightly less than your value under conditioning. This, however, is not the average percentage of loss covered; it's the average loss covered, expressed as a percentage of the average loss incurred. The average percentage of loss covered is



                    $$
                    left(1-mathrm e^-0.2right)cdot0+mathrm e^-0.2int_0^inftyfrac xx+0.2,mathrm e^-xmathrm dx=0.2operatornameEi(-0.2)+mathrm e^-0.2approx57.4%;,
                    $$



                    where $operatornameEi$ denotes the exponential integral.







                    share|cite|improve this answer













                    share|cite|improve this answer



                    share|cite|improve this answer











                    answered Jul 20 at 7:02









                    joriki

                    164k10180328




                    164k10180328











                    • Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                      – Marcel
                      Jul 20 at 7:21










                    • @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                      – joriki
                      Jul 20 at 7:28

















                    • Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                      – Marcel
                      Jul 20 at 7:21










                    • @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                      – joriki
                      Jul 20 at 7:28
















                    Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                    – Marcel
                    Jul 20 at 7:21




                    Thanks for the answer, I was somehow only thinking a "loss" had occurred if the insurance company was required to pay for it. I suppose I was not thinking from the perspective of the person who suffered the loss! It is not obvious to me how you obtained your last series of equations. Is "average percentage of loss covered" a standard formula?
                    – Marcel
                    Jul 20 at 7:21












                    @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                    – joriki
                    Jul 20 at 7:28





                    @Marcel: A better term would actually have been "expected percentage of loss". It's just the expected value of the percentage of loss. The percentage of loss is $0$ with probability $1-mathrm e^-0.2$ (loss below the deductible) and $x/(x+0.2)$ if the insurance pays $x$ for a loss of $x+0.2$, with probability density $mathrm e^-(x+0.2)$. I got the exponential integral from Wolfram|Alpha; you can get it by transforming to $u=x+0.2$ and decomposing the fraction into $1+0.2/u$ (I should have written it in terms of the loss $u$ instead of the coverage $x$ in the first place.)
                    – joriki
                    Jul 20 at 7:28













                     

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