|
325 | 325 | "source": [ |
326 | 326 | "## Consumption Function Analysis\n", |
327 | 327 | "\n", |
328 | | - "The first set of figures will focus on the core of the consumption-saving problem: the consumption function $\\cFunc(\\mNrm)$, which maps market resources $\\mNrm$ to consumption. We will demonstrate the extrapolation problem inherent in the standard EGM and show how the Method of Moderation resolves it by respecting theoretical bounds.\n", |
| 328 | + "The first set of figures will focus on the core of the consumption-saving problem: the consumption function $\\mathbf{c}(m)$, which maps market resources $m$ to consumption. We will demonstrate the extrapolation problem inherent in the standard EGM and show how the Method of Moderation resolves it by respecting theoretical bounds.\n", |
329 | 329 | "\n", |
330 | 330 | "### Figure 1: The EGM Extrapolation Problem\n", |
331 | 331 | "\n", |
|
432 | 432 | "metadata": {}, |
433 | 433 | "source": [ |
434 | 434 | "```{important} Key Theoretical Insight\n", |
435 | | - "The constraint $\\cFuncPes(\\mNrm) \\leq \\cFuncReal(\\mNrm) \\leq \\cFuncOpt(\\mNrm)$ is the foundation for MoM {cite:p}`CarrollShanker2024`. The lower bound arises from the natural borrowing constraint in incomplete markets models {cite:p}`Aiyagari1994,Huggett1993,Zeldes1989,Deaton1991`.\n", |
| 435 | + "The constraint $\\underline{\\mathbf{c}}(m) \\leq \\hat{\\mathbf{c}}(m) \\leq \\bar{\\mathbf{c}}(m)$ is the foundation for MoM {cite:p}`CarrollShanker2024`. The lower bound arises from the natural borrowing constraint in incomplete markets models {cite:p}`Aiyagari1994,Huggett1993,Zeldes1989,Deaton1991`.\n", |
436 | 436 | "```\n", |
437 | 437 | "\n", |
438 | 438 | "### Figure 3: Method of Moderation Solution\n", |
|
494 | 494 | "\n", |
495 | 495 | "MoM steps (notation matches the paper):\n", |
496 | 496 | "1. Solve standard EGM for realist consumption at gridpoints\n", |
497 | | - "2. Transform to $\\logmNrmEx = \\log(\\mNrm - \\mNrmMin)$\n", |
498 | | - "3. Compute $\\modRte(\\logmNrmEx) = (\\cFuncReal - \\cFuncPes)/(\\cFuncOpt - \\cFuncPes) \\in [0,1]$ (Eq. {eq}`eq:modRte`)\n", |
499 | | - "4. Apply logit: $\\logitModRte = \\log(\\modRte/(1-\\modRte))$\n", |
500 | | - "5. Interpolate $\\logitModRte(\\logmNrmEx)$ with derivatives\n", |
501 | | - "6. Reconstruct: $\\cFuncReal = \\cFuncPes + \\modRte \\cdot (\\cFuncOpt - \\cFuncPes)$\n", |
| 497 | + "2. Transform to $\\boldsymbol{\\mu} = \\log(m - \\underline{m})$\n", |
| 498 | + "3. Compute $\\boldsymbol{\\omega}(\\boldsymbol{\\mu}) = (\\hat{\\mathbf{c}} - \\underline{\\mathbf{c}})/(\\bar{\\mathbf{c}} - \\underline{\\mathbf{c}}) \\in [0,1]$ (Eq. {eq}`eq:modRte`)\n", |
| 499 | + "4. Apply logit: $\\boldsymbol{\\chi} = \\log(\\boldsymbol{\\omega}/(1-\\boldsymbol{\\omega}))$\n", |
| 500 | + "5. Interpolate $\\boldsymbol{\\chi}(\\boldsymbol{\\mu})$ with derivatives\n", |
| 501 | + "6. Reconstruct: $\\hat{\\mathbf{c}} = \\underline{\\mathbf{c}} + \\boldsymbol{\\omega} \\cdot (\\bar{\\mathbf{c}} - \\underline{\\mathbf{c}})$\n", |
502 | 502 | "\n", |
503 | 503 | "This ensures bound compliance via asymptotically linear extrapolation, as derived in the paper. HARK uses **cubic Hermite interpolation** {cite:p}`Fritsch1980,FritschButland1984` for accuracy; see {cite:p}`Santos2000,JuddMaliarMaliar2017` on function approximation and error bounding.\n", |
504 | 504 | "```\n", |
505 | 505 | "\n", |
506 | 506 | "```{note} The Transformation\n", |
507 | | - "The logit maps $\\modRte \\in (0,1)$ to $\\logitModRte \\in (-\\infty, +\\infty)$ and becomes asymptotically linear with positive slope $\\logitModRteMu > 0$ as wealth increases.\n", |
| 507 | + "The logit maps $\\boldsymbol{\\omega} \\in (0,1)$ to $\\boldsymbol{\\chi} \\in (-\\infty, +\\infty)$ and becomes asymptotically linear with positive slope $\\partial \\boldsymbol{\\chi} / \\partial \\boldsymbol{\\mu} > 0$ as wealth increases.\n", |
508 | 508 | "```\n", |
509 | 509 | "\n", |
510 | 510 | "### Figure 4: MoM Consumption Function\n", |
511 | 511 | "\n", |
512 | | - "[](#fig:mom-consumption-function) ({ref}`Figure 4 <fig:IntExpFOCInvPesReaOptNeed45>` in the paper) shows MoM consumption between optimist and pessimist bounds, plus a **tighter upper bound** derived from $\\MPCmax$ near the borrowing constraint {cite:p}`Carroll2001MPCBound,MaToda2021SavingRateRich,CarrollToche2009`. The cusp intersection is given by {eq}`eq:mNrmCusp`." |
| 512 | + "[](#fig:mom-consumption-function) ({ref}`Figure 4 <fig:IntExpFOCInvPesReaOptNeed45>` in the paper) shows MoM consumption between optimist and pessimist bounds, plus a **tighter upper bound** derived from $\\bar{\\boldsymbol{\\kappa}}$ near the borrowing constraint {cite:p}`Carroll2001MPCBound,MaToda2021SavingRateRich,CarrollToche2009`. The cusp intersection is given by {eq}`eq:mNrmCusp`." |
513 | 513 | ] |
514 | 514 | }, |
515 | 515 | { |
|
608 | 608 | "\n", |
609 | 609 | "## Method of Moderation Framework\n", |
610 | 610 | "\n", |
611 | | - "### Figure 6: Moderation Ratio Function $\\modRte(\\mNrm)$\n", |
| 611 | + "### Figure 6: Moderation Ratio Function $\\boldsymbol{\\omega}(m)$\n", |
612 | 612 | "\n", |
613 | 613 | "::::{admonition} Definition: The Moderation Ratio\n", |
614 | 614 | ":class: note\n", |
615 | 615 | "\n", |
616 | 616 | "The **moderation ratio** (Eq. {eq}`eq:modRte`):\n", |
617 | 617 | "\n", |
618 | 618 | "$$\n", |
619 | | - "\\modRte(\\mNrm) = \\frac{\\cFuncReal(\\mNrm) - \\cFuncPes(\\mNrm)}{\\cFuncOpt(\\mNrm) - \\cFuncPes(\\mNrm)} \\in (0,1)\n", |
| 619 | + "\\boldsymbol{\\omega}(m) = \\frac{\\hat{\\mathbf{c}}(m) - \\underline{\\mathbf{c}}(m)}{\\bar{\\mathbf{c}}(m) - \\underline{\\mathbf{c}}(m)} \\in (0,1)\n", |
620 | 620 | "$$\n", |
621 | 621 | "\n", |
622 | | - "This ratio is strictly between 0 and 1 due to prudence {cite:p}`CarrollKimball1996`. At $\\modRte = 0$ the realist behaves like the pessimist (maximum precautionary saving); at $\\modRte = 1$ like the optimist (no precautionary saving). [](#fig:moderation-ratio) plots this ratio across wealth levels.\n", |
| 622 | + "This ratio is strictly between 0 and 1 due to prudence {cite:p}`CarrollKimball1996`. At $\\boldsymbol{\\omega} = 0$ the realist behaves like the pessimist (maximum precautionary saving); at $\\boldsymbol{\\omega} = 1$ like the optimist (no precautionary saving). [](#fig:moderation-ratio) plots this ratio across wealth levels.\n", |
623 | 623 | "::::" |
624 | 624 | ] |
625 | 625 | }, |
|
665 | 665 | "id": "587c8ea3", |
666 | 666 | "metadata": {}, |
667 | 667 | "source": [ |
668 | | - "```{note} Economic Interpretation of $\\modRte(\\mNrm)$\n", |
669 | | - "$\\modRte \\to 1$ at high wealth (approaches optimist), $\\modRte \\to 0$ at low wealth (approaches pessimist). This monotonic pattern ensures proper economic behavior across the wealth distribution.\n", |
| 668 | + "```{note} Economic Interpretation of $\\boldsymbol{\\omega}(m)$\n", |
| 669 | + "$\\boldsymbol{\\omega} \\to 1$ at high wealth (approaches optimist), $\\boldsymbol{\\omega} \\to 0$ at low wealth (approaches pessimist). This monotonic pattern ensures proper economic behavior across the wealth distribution.\n", |
670 | 670 | "```\n", |
671 | 671 | "\n", |
672 | 672 | "### Figure 7: The Logit Transformation\n", |
|
677 | 677 | "The **logit transformation** (Eq. {eq}`eq:chi`) maps the bounded ratio to an unbounded space:\n", |
678 | 678 | "\n", |
679 | 679 | "$$\n", |
680 | | - "\\logitModRte(\\logmNrmEx) = \\log\\left(\\frac{\\modRte(\\logmNrmEx)}{1 - \\modRte(\\logmNrmEx)}\\right)\n", |
| 680 | + "\\boldsymbol{\\chi}(\\boldsymbol{\\mu}) = \\log\\left(\\frac{\\boldsymbol{\\omega}(\\boldsymbol{\\mu})}{1 - \\boldsymbol{\\omega}(\\boldsymbol{\\mu})}\\right)\n", |
681 | 681 | "$$\n", |
682 | 682 | "\n", |
683 | | - "where $\\logmNrmEx = \\log(\\mNrm - \\mNrmMin)$. As [](#fig:logit-transformation) shows, $\\logitModRte$ is nearly linear, making it well-suited for interpolation.\n", |
| 683 | + "where $\\boldsymbol{\\mu} = \\log(m - \\underline{m})$. As [](#fig:logit-transformation) shows, $\\boldsymbol{\\chi}$ is nearly linear, making it well-suited for interpolation.\n", |
684 | 684 | "::::" |
685 | 685 | ] |
686 | 686 | }, |
|
726 | 726 | "metadata": {}, |
727 | 727 | "source": [ |
728 | 728 | "```{important} Why Asymptotic Linearity Matters\n", |
729 | | - "As $\\logmNrmEx \\to \\infty$, $\\logitModRte$ becomes linear with slope $\\logitModRteMu > 0$. This prevents extrapolation errors, ensures smooth convergence to the optimist bound, and maintains numerical stability.\n", |
| 729 | + "As $\\boldsymbol{\\mu} \\to \\infty$, $\\boldsymbol{\\chi}$ becomes linear with slope $\\partial \\boldsymbol{\\chi} / \\partial \\boldsymbol{\\mu} > 0$. This prevents extrapolation errors, ensures smooth convergence to the optimist bound, and maintains numerical stability.\n", |
730 | 730 | "```\n", |
731 | 731 | "\n", |
732 | | - "```{note} Properties of $\\logitModRte(\\logmNrmEx)$\n", |
733 | | - "Unbounded domain $(-\\infty, \\infty)$, monotonically increasing, asymptotically linear. $\\logitModRte > 0$ indicates behavior closer to optimist; $\\logitModRte < 0$ closer to pessimist.\n", |
| 732 | + "```{note} Properties of $\\boldsymbol{\\chi}(\\boldsymbol{\\mu})$\n", |
| 733 | + "Unbounded domain $(-\\infty, \\infty)$, monotonically increasing, asymptotically linear. $\\boldsymbol{\\chi} > 0$ indicates behavior closer to optimist; $\\boldsymbol{\\chi} < 0$ closer to pessimist.\n", |
734 | 734 | "```\n", |
735 | 735 | "\n", |
736 | 736 | "## Function Properties and Bounds\n", |
737 | 737 | "\n", |
738 | 738 | "### Figure 8: MoM MPC Bounded by Theory\n", |
739 | 739 | "\n", |
740 | | - "The **MPC** ($\\partial \\cNrm / \\partial \\mNrm$) is bounded between $\\MPCmin$ (optimist) and $\\MPCmax$ (at the borrowing constraint), as detailed in {ref}`the paper <a-tighter-upper-bound>` and Eq. {eq}`eq:MPCModeration` {cite:p}`Carroll2001MPCBound`. [](#fig:mpc-bounds) confirms MoM respects these bounds.\n", |
| 740 | + "The **MPC** ($\\partial c / \\partial m$) is bounded between $\\underline{\\boldsymbol{\\kappa}}$ (optimist) and $\\bar{\\boldsymbol{\\kappa}}$ (at the borrowing constraint), as detailed in {ref}`the paper <a-tighter-upper-bound>` and Eq. {eq}`eq:MPCModeration` {cite:p}`Carroll2001MPCBound`. [](#fig:mpc-bounds) confirms MoM respects these bounds.\n", |
741 | 741 | "\n", |
742 | 742 | "```{tip} Policy Applications\n", |
743 | 743 | "Bounded MPC estimates prevent nonsensical policy multipliers in DSGE models. MoM ensures economically meaningful MPCs for policy analysis.\n", |
|
785 | 785 | "metadata": {}, |
786 | 786 | "source": [ |
787 | 787 | "```{hint} MPC Economic Interpretation\n", |
788 | | - "MoM MPC declines with $\\mNrm$: poor consumers spend windfalls immediately (MPC $\\to \\MPCmax$), wealthy consumers save them (MPC $\\to \\MPCmin$), reflecting diminishing marginal utility.\n", |
| 788 | + "MoM MPC declines with $m$: poor consumers spend windfalls immediately (MPC $\\to \\bar{\\boldsymbol{\\kappa}}$), wealthy consumers save them (MPC $\\to \\underline{\\boldsymbol{\\kappa}}$), reflecting diminishing marginal utility.\n", |
789 | 789 | "```\n", |
790 | 790 | "\n", |
791 | 791 | "### Figure 9: Value Functions Bounded by Theory\n", |
792 | 792 | "\n", |
793 | | - "The **value function** $\\vFunc(\\mNrm)$ is also bounded by optimist and pessimist solutions {cite:p}`Aiyagari1994,Huggett1993`. [](#fig:value-functions) compares truth, EGM, and MoM value functions." |
| 793 | + "The **value function** $\\mathbf{v}(m)$ is also bounded by optimist and pessimist solutions {cite:p}`Aiyagari1994,Huggett1993`. [](#fig:value-functions) compares truth, EGM, and MoM value functions." |
794 | 794 | ] |
795 | 795 | }, |
796 | 796 | { |
|
843 | 843 | "Uncertainty matters most at low wealth where buffers are small; the optimist-pessimist gap narrows with wealth as assets provide natural insurance.\n", |
844 | 844 | "```\n", |
845 | 845 | "\n", |
846 | | - "### Figure 10: Inverse Value Functions $\\vInv(\\mNrm)$\n", |
| 846 | + "### Figure 10: Inverse Value Functions ${\\scriptsize \\boldsymbol{\\Lambda}}(m)$\n", |
847 | 847 | "\n", |
848 | | - "The **inverse value function** $\\vInv(\\mNrm) = \\uFunc^{-1}(\\vFunc(\\mNrm))$ gives the consumption equivalent of lifetime utility. It is more linear than $\\vFunc(\\mNrm)$ near the borrowing constraint, making it better suited for interpolation. [](#fig:inverse-value-functions) compares the three solutions." |
| 848 | + "The **inverse value function** ${\\scriptsize \\boldsymbol{\\Lambda}}(m) = \\mathbf{u}^{-1}(\\mathbf{v}(m))$ gives the consumption equivalent of lifetime utility. It is more linear than $\\mathbf{v}(m)$ near the borrowing constraint, making it better suited for interpolation. [](#fig:inverse-value-functions) compares the three solutions." |
849 | 849 | ] |
850 | 850 | }, |
851 | 851 | { |
|
892 | 892 | "metadata": {}, |
893 | 893 | "source": [ |
894 | 894 | "```{note} Inverse Value Function Interpretation\n", |
895 | | - "The inverse transformation converts utility to consumption units: $\\vInv(5) = 0.8$ means 5 units of wealth provides lifetime utility equivalent to consuming 0.8 forever. HARK uses this more linear representation for interpolation.\n", |
| 895 | + "The inverse transformation converts utility to consumption units: ${\\scriptsize \\boldsymbol{\\Lambda}}(5) = 0.8$ means 5 units of wealth provides lifetime utility equivalent to consuming 0.8 forever. HARK uses this more linear representation for interpolation.\n", |
896 | 896 | "```\n", |
897 | 897 | "\n", |
898 | 898 | "### Figure 11: Value Function Moderation Ratio\n", |
899 | 899 | "\n", |
900 | 900 | "MoM applies to any bounded function. The **inverse value function moderation ratio** (Eq. {eq}`eq:valModRteReal`):\n", |
901 | 901 | "\n", |
902 | 902 | "$$\n", |
903 | | - "\\valModRte(\\mNrm) = \\frac{\\vInvReal(\\mNrm) - \\vInvPes(\\mNrm)}{\\vInvOpt(\\mNrm) - \\vInvPes(\\mNrm)}\n", |
| 903 | + "\\boldsymbol{\\Omega}(m) = \\frac{\\hat{{\\scriptsize \\boldsymbol{\\Lambda}}}(m) - \\underline{{\\scriptsize \\boldsymbol{\\Lambda}}}(m)}{\\bar{{\\scriptsize \\boldsymbol{\\Lambda}}}(m) - \\underline{{\\scriptsize \\boldsymbol{\\Lambda}}}(m)}\n", |
904 | 904 | "$$\n", |
905 | 905 | "\n", |
906 | 906 | "follows the same pattern as the consumption ratio, as shown in [](#fig:value-moderation-ratio)." |
|
949 | 949 | "metadata": {}, |
950 | 950 | "source": [ |
951 | 951 | "```{hint} Value Function Moderation Interpretation\n", |
952 | | - "$\\valModRte \\to 1$ at high wealth (low uncertainty cost), $\\valModRte \\to 0$ at low wealth (high uncertainty cost). The pattern confirms uncertainty's welfare cost diminishes with wealth.\n", |
| 952 | + "$\\boldsymbol{\\Omega} \\to 1$ at high wealth (low uncertainty cost), $\\boldsymbol{\\Omega} \\to 0$ at low wealth (high uncertainty cost). The pattern confirms uncertainty's welfare cost diminishes with wealth.\n", |
953 | 953 | "```\n", |
954 | 954 | "\n", |
955 | 955 | "### Figure 12: Cusp Point Visualization\n", |
956 | 956 | "\n", |
957 | 957 | "The **cusp point** (Eq. {eq}`eq:mNrmCusp`) is where optimist and tighter upper bounds intersect:\n", |
958 | 958 | "\n", |
959 | 959 | "$$\n", |
960 | | - "\\mNrmCusp = \\mNrmMin + \\frac{\\MPCmin \\cdot \\hNrmEx}{\\MPCmax - \\MPCmin}\n", |
| 960 | + "m^* = \\underline{m} + \\frac{\\underline{\\boldsymbol{\\kappa}} \\cdot \\Delta h}{\\bar{\\boldsymbol{\\kappa}} - \\underline{\\boldsymbol{\\kappa}}}\n", |
961 | 961 | "$$\n", |
962 | 962 | "\n", |
963 | | - "Below the cusp, the tighter bound ($\\MPCmax$ slope) constrains; above, the optimist bound constrains. See `IndShockMoMCuspConsumerType` for the three-piece implementation." |
| 963 | + "Below the cusp, the tighter bound ($\\bar{\\boldsymbol{\\kappa}}$ slope) constrains; above, the optimist bound constrains. See `IndShockMoMCuspConsumerType` for the three-piece implementation." |
964 | 964 | ] |
965 | 965 | }, |
966 | 966 | { |
|
1005 | 1005 | "metadata": {}, |
1006 | 1006 | "source": [ |
1007 | 1007 | "```{hint} Cusp Point Interpretation\n", |
1008 | | - "Below cusp: MPC near $\\MPCmax$, tighter bound constrains. Above cusp: behavior approaches optimist ($\\MPCmin$), optimist bound constrains. The envelope is the minimum of both bounds.\n", |
| 1008 | + "Below cusp: MPC near $\\bar{\\boldsymbol{\\kappa}}$, tighter bound constrains. Above cusp: behavior approaches optimist ($\\underline{\\boldsymbol{\\kappa}}$), optimist bound constrains. The envelope is the minimum of both bounds.\n", |
1009 | 1009 | "```\n", |
1010 | 1010 | "\n", |
1011 | 1011 | "## Further Extensions: Stochastic Rate of Return\n", |
|
1070 | 1070 | "metadata": {}, |
1071 | 1071 | "source": [ |
1072 | 1072 | "```{hint} Stochastic Returns Interpretation\n", |
1073 | | - "Deterministic optimist uses $\\MPCmin = 1 - (\\DiscFac \\Rfree)^{1/\\CRRA}$; stochastic optimist uses $\\MPCmin = 1 - (\\DiscFac \\Ex[\\Risky^{1-\\CRRA}])^{1/\\CRRA}$. Return uncertainty raises MPC and narrows the feasible region. See {ref}`stochastic-returns-mgf-derivation` for the MGF derivation.\n", |
| 1073 | + "Deterministic optimist uses $\\underline{\\boldsymbol{\\kappa}} = 1 - (\\beta \\text{R})^{1/\\rho}$; stochastic optimist uses $\\underline{\\boldsymbol{\\kappa}} = 1 - (\\beta \\mathbf{\\mathbb{E}}[\\mathbf{R}^{1-\\rho}])^{1/\\rho}$. Return uncertainty raises MPC and narrows the feasible region. See {ref}`stochastic-returns-mgf-derivation` for the MGF derivation.\n", |
1074 | 1074 | "```\n", |
1075 | 1075 | "\n", |
1076 | 1076 | "## Summary\n", |
|
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