# Asymmetric Poisson Pseudo-Maximum Likelihood (APPML)

    library(capybara)

## Overview

Standard Poisson Pseudo-Maximum Likelihood (PPML) estimates the conditional mean of the outcome variable. `fepoisson_asymmetric()` extends this by fitting conditional expectiles — a generalization that lets you examine different parts of the conditional distribution with the same efficiency gains of high-dimensional fixed effects.

The implementation follows Bergstrand et al. (2025): instead of minimizing a symmetric loss, the estimator minimizes an asymmetric weighted loss controlled by a single parameter `expectile` (\(\tau \in (0, 1)\)):

- \(\tau = 0.5\) — standard PPML (symmetric, estimates the conditional mean).
- \(\tau < 0.5\) — places more weight on observations with negative residuals, pulling estimates toward the lower part of the distribution.
- \(\tau > 0.5\) — places more weight on observations with positive residuals, shifting estimates toward the upper part of the distribution.

The `expectile` is set through `fit_control()` and defaults to `0.5`, so if you don’t specify it, `fepoisson_asymmetric()` will behave like standard PPML.

## Estimating lower and upper expectiles

Shifting `expectile` away from 0.5 lets you trace how the entire conditional distribution of `trade` varies with `ldist` and other predictors, after absorbing the exporter-year and importer-year fixed effects.

    ross2004_subset <- ross2004[ross2004$year %in% seq(1989, 1999, 5), ]
    ross2004_subset$trade <- exp(ross2004_subset$ltrade)
    ross2004_subset$exp_year <- paste0(ross2004_subset$ctry1, ross2004_subset$year)
    ross2004_subset$imp_year <- paste0(ross2004_subset$ctry2, ross2004_subset$year)

    # 10th expectile — sensitive to small values of trade
    fit10 <- fepoisson_asymmetric(
      trade ~ ldist + border + comlang + colony | exp_year + imp_year,
      data    = ross2004_subset,
      control = fit_control(expectile = 0.1)
    )

    summary(fit10)

    Formula: trade ~ ldist + border + comlang + colony | exp_year + imp_year

    Family: Poisson

    Estimates:

    |         | Estimate | Std. Error | z value     | Pr(>|z|)  |
    |---------|----------|------------|-------------|-----------|
    | ldist   |  -1.0916 |     0.0000 | -39358.0438 | 0.0000 ** |
    | border  |   0.3419 |     0.0001 |   5729.9222 | 0.0000 ** |
    | comlang |   0.3352 |     0.0001 |   5700.9311 | 0.0000 ** |
    | colony  |   0.3942 |     0.0001 |   5416.8894 | 0.0000 ** |

    Significance codes: ** p < 0.01; * p < 0.05; + p < 0.10

    Fixed effects:
      exp_year: 457
      imp_year: 457

    Number of observations: Full 21450; Missing 0; Perfect classification 0

    Number of Fisher Scoring iterations: 17

    # 90th expectile — sensitive to large values of trade
    fit90 <- fepoisson_asymmetric(
      trade ~ ldist + border + comlang + colony | exp_year + imp_year,
      data    = ross2004_subset,
      control = fit_control(expectile = 0.9)
    )

    summary(fit90)

    Formula: trade ~ ldist + border + comlang + colony | exp_year + imp_year

    Family: Poisson

    Estimates:

    |         | Estimate | Std. Error | z value     | Pr(>|z|)  |
    |---------|----------|------------|-------------|-----------|
    | ldist   |  -0.9096 |     0.0000 | -45409.0443 | 0.0000 ** |
    | border  |   0.3160 |     0.0000 |   7382.8698 | 0.0000 ** |
    | comlang |   0.1897 |     0.0000 |   4789.5546 | 0.0000 ** |
    | colony  |   0.4657 |     0.0001 |   7910.0068 | 0.0000 ** |

    Significance codes: ** p < 0.01; * p < 0.05; + p < 0.10

    Fixed effects:
      exp_year: 457
      imp_year: 457

    Number of observations: Full 21450; Missing 0; Perfect classification 0

    Number of Fisher Scoring iterations: 18

## Comparing coefficients across expectiles

A natural use-case is comparing how the effect of a regressor changes across the distribution. The table below collects the coefficient on `wt` at the three expectile levels:

    summary_table(
      fit10, fit90,
      model_names = c("10th expectile", "90th expectile")
    )

    |     Variable     |   10th expectile   |   90th expectile   |
    |------------------|--------------------|--------------------|
    | ldist            |           -1.092** |           -0.910** |
    |                  |            (0.000) |            (0.000) |
    | border           |            0.342** |            0.316** |
    |                  |            (0.000) |            (0.000) |
    | comlang          |            0.335** |            0.190** |
    |                  |            (0.000) |            (0.000) |
    | colony           |            0.394** |            0.466** |
    |                  |            (0.000) |            (0.000) |
    |                  |                    |                    |
    | Fixed effects    |                    |                    |
    | exp_year         |                Yes |                Yes |
    | imp_year         |                Yes |                Yes |
    |                  |                    |                    |
    | N                |             21,450 |             21,450 |

    Standard errors in parenthesis
    Significance levels: ** p < 0.01; * p < 0.05; + p < 0.10

The log-distance coefficient shrinks in magnitude from `fit10` to `fit90`, which indicates that trade much less with each other at the top of the distribution compared to the bottom of the distribution.

## Convergence diagnostics

The outer APPML loop updates observation weights until the coefficient vector stops changing. You can inspect convergence through the returned list elements:

    cat("Outer loop converged:", fit10$conv_outer, "\n")
    cat("Outer iterations:    ", fit10$iter_outer, "\n")
    cat("Expectile used:      ", fit10$expectile, "\n")

    Outer loop converged: TRUE
    Outer iterations:     4
    Expectile used:       0.1

## References

Bergstrand, Jeffrey H., Matthew W. Clance, and JMC Santos Silva. “The tails of gravity: Using expectiles to quantify the trade-margins effects of economic integration agreements.” Journal of International Economics (2025): 104145.