# Choice-based sampling and WESML weights

Choice data are often sampled by outcome. A transport researcher running an on-site survey interviews travellers at the terminal of the mode they actually chose; a hospital-choice study may oversample patients of rare hospitals; a marketing team may recruit equal numbers of buyers of each brand. In each case the unit is drawn conditional on the alternative it chose, so the sample choice shares are not the population choice shares. Treating such a sample as random changes the likelihood target and, in general, biases the estimates.

WESML fixes that sampling problem; it does not fix every econometric problem. The weighted likelihood still relies on the maintained utility specification and on whatever exogeneity assumptions justify interpreting the covariates, especially prices, as demand shifters rather than equilibrium outcomes.

Manski and Lerman’s (1977) weighted exogenous sample maximum likelihood (WESML) correction weights each choice situation by

\[ w_i = \frac{Q_{j(i)}}{H_{j(i)}}, \]

where \(j(i)\) is the alternative chosen by situation \(i\), \(Q_j\) is the population share choosing alternative \(j\), and \(H_j\) is the corresponding sample share. Under the maintained choice model, exogenous within-stratum sampling, correct population shares, and common support, maximizing the weighted log-likelihood \(\sum_i w_i \log P_i\) targets the population parameters. choicer provides two helpers:

- `sample_by_choice()` draws a choice-based sample from a population frame and attaches WESML weights.
- `wesml_weights()` computes the same weights when you already have a sample and know the population shares `Q`.

Both helpers normalize the weights to mean 1 by default. Normalization — and indeed any rescaling of the weights by a common factor — leaves the point estimates and the robust (sandwich) variance unchanged, so the attached `.wesml_weight` need not equal \(Q/H\) literally; only the relative weights across strata matter.

Before fitting, record where `Q` came from, its population and reference period, how its alternative definitions map to the estimation sample, and whether the totals themselves are estimated. `wesml_weights()` treats `Q` as fixed: it does not propagate sampling error, benchmarking error, or uncertainty from an external survey into the covariance matrix. If that uncertainty is material, recompute the complete fit over plausible or resampled `Q` values and report how the coefficients and policy objects move.

    library(choicer)
    library(data.table)
    #>
    #> Attaching package: 'data.table'
    #> The following object is masked from 'package:base':
    #>
    #>     %notin%
    set_num_threads(2)

## Build a population

For exposition, start from a simulated population in which tastes are heterogeneous (a random coefficient on `w1` and `w2`), so a mixed logit is the natural estimator. We turn off the outside option and fix the choice set so that every situation has exactly one chosen alternative and the strata are clean. In empirical work the population shares `Q` usually come from administrative totals, market shares, or survey weights external to the choice-based estimation sample.

    sim <- simulate_mxl_data(
      N               = 3000,
      J               = 4,
      Sigma           = diag(c(1.0, 1.5)),  # two uncorrelated random coefficients
      seed            = 11,
      outside_option  = FALSE,
      vary_choice_set = FALSE
    )

    pop <- as.data.table(sim$data)
    Q <- prop.table(table(pop[choice == 1, alt]))
    round(Q, 3)
    #>
    #>     1     2     3     4
    #> 0.355 0.150 0.330 0.165

## Draw a choice-based sample

Now sample the same number of choice situations from each chosen alternative. This keeps whole choice situations together: if an id is sampled, all of its alternative rows are retained.

    cb <- sample_by_choice(
      pop,
      id_col     = "id",
      alt_col    = "alt",
      choice_col = "choice",
      n_per_alt  = 300L,
      seed       = 12L
    )

    strata <- sort(names(attr(cb, "Q")))
    rbind(
      population = attr(cb, "Q")[strata],
      sample     = attr(cb, "H")[strata]
    ) |> round(3)
    #>                1    2    3     4
    #> population 0.355 0.15 0.33 0.165
    #> sample     0.250 0.25 0.25 0.250

    cb[choice == 1, .(id, chosen_alt = alt, .wesml_weight)][1:8]
    #>       id chosen_alt .wesml_weight
    #>    <int>      <int>         <num>
    #> 1:     1          2        0.6000
    #> 2:     4          1        1.4200
    #> 3:     7          1        1.4200
    #> 4:     8          1        1.4200
    #> 5:    10          3        1.3187
    #> 6:    11          2        0.6000
    #> 7:    12          4        0.6613
    #> 8:    13          3        1.3187

The sample choice shares are deliberately equalized, but the attached weights restore the population shares in the weighted likelihood. The weight is constant within an id and repeated across that id’s alternative rows, which is exactly the row-level layout `run_mxlogit()` — and equally `run_mnlogit()` / `run_nestlogit()` — expects through `weights_col`.

## Weighted estimation and inference

We fit two mixed logits on the choice-based sample: an ordinary (unweighted) fit that ignores the sampling design, and a WESML fit that passes the weight column and requests the robust sandwich covariance. Passing `weights_col` by name keeps the estimation target visible in the script, which is the recommended style even when the data already carry a `choice_sampling` attribute from `sample_by_choice()`.

    common <- list(
      data            = cb,
      id_col          = "id",
      alt_col         = "alt",
      choice_col      = "choice",
      covariate_cols  = c("x1", "x2"),  # fixed coefficients
      random_var_cols = c("w1", "w2"),  # random coefficients
      S               = 100L,
      draws           = "generate",
      seed            = 7L,
      scale_vars      = "sd"
    )

    # sample_by_choice() records WESML provenance, and choicer deliberately applies
    # its attached weights automatically. Strip that provenance on a copy to create
    # the deliberately misspecified unweighted benchmark.
    cb_unweighted <- copy(cb)
    attr(cb_unweighted, "choice_sampling") <- NULL
    common_unweighted <- common
    common_unweighted$data <- cb_unweighted

    fit_unweighted <- do.call(
      run_mxlogit,
      c(common_unweighted, list(se_method = "bhhh"))
    )
    #> Optimization run time 0h:0m:0.23s

    fit_wesml <- do.call(run_mxlogit, c(common, list(
      weights_col = ".wesml_weight",
      se_method   = "sandwich"
    )))
    #> Optimization run time 0h:0m:0.25s

> Tip. As in the mixed logit vignette, raise the number of draws `S` until the estimates are stable and warm-start a stubborn solver with `theta_init`. `S = 100` here keeps the package build quick.

The unweighted estimator treats the equalized sample shares as if they were the population shares; WESML reweights the sampled situations back to the population. With alternative-specific constants in the model the correction is most visible in the constants and, through them, in the fitted shares:

    round(cbind(
      unweighted = coef(fit_unweighted),
      wesml      = coef(fit_wesml)
    ), 3)
    #>       unweighted  wesml
    #> x1         0.772  0.810
    #> x2        -0.570 -0.561
    #> L_11       0.036  0.169
    #> L_22       0.097  0.127
    #> ASC_2     -0.117 -1.107
    #> ASC_3     -0.003 -0.080
    #> ASC_4     -0.093 -0.978

    share_compare <- rbind(
      population = as.numeric(Q),
      wesml      = drop(predict(fit_wesml, type = "shares")),
      unweighted = drop(predict(fit_unweighted, type = "shares"))
    )
    colnames(share_compare) <- names(Q)
    round(share_compare, 3)
    #>                1     2     3     4
    #> population 0.355 0.150 0.330 0.165
    #> wesml      0.358 0.149 0.330 0.163
    #> unweighted 0.255 0.246 0.252 0.246

Here `predict(..., type = "shares")` uses each fit’s stored aggregation weights. The WESML-weighted fitted shares therefore track the population shares `Q`, while the unweighted fit tracks the equalized sample shares — a direct picture of the bias the correction removes. Exact equality is not required in a finite sample with simulated probabilities and numerical optimization. In a single finite sample the WESML estimates need not be closer to the truth parameter by parameter, but they target the population likelihood under the choice-based sampling design.

For inference, the point of `se_method = "sandwich"` is that under non-uniform weights the inverse weighted Hessian and the ordinary BHHH variance are not valid covariance estimators. The sandwich uses the weighted Hessian as bread, \(A = \sum_i w_i(-H_i)\), and the weight-squared outer product of the per-situation scores as meat, \(B = \sum_i w_i^2 s_i s_i'\), giving \(V = A^{-1} B A^{-1}\). Because \(A\) scales linearly and \(B\) quadratically in the weights, \(V\) is invariant to any common rescaling of them — consistent with the mean-1 normalization above.

    summary(fit_wesml)
    #> Mixed Logit (MXL) model
    #>
    #> Parameter    Estimate  Std.Error  z-value  Pr(>|z|)
    #> x1           0.809770   0.080489  10.0607  0.00e+00  ***
    #> x2          -0.561483   0.075121  -7.4743  7.75e-14  ***
    #> Sigma_11     1.402991   0.539452   2.6008  9.30e-03  **
    #> Sigma_22     1.289004   0.533570   2.4158  1.57e-02  *
    #> ASC_2       -1.107296   0.107727 -10.2787  0.00e+00  ***
    #> ASC_3       -0.079835   0.101007  -0.7904  4.29e-01
    #> ASC_4       -0.978022   0.107984  -9.0571  0.00e+00  ***
    #> ---
    #> Signif. codes:  '***' 0.001 '**' 0.01 '*' 0.05
    #>
    #> Random coefficient covariance (Sigma):
    #>       w1    w2
    #> w1 1.403 0.000
    #> w2 0.000 1.289
    #>
    #> Std. Errors: Sandwich (robust)
    #> Weighting: WESML choice-based
    #> Log-likelihood: -1471.18
    #> AIC: 2956.36  | BIC: 2991.99
    #> McFadden R2: 0.116 (adj: 0.111) | Hit rate: 0.436
    #> N: 1200  | Parameters: 7
    #> Optimization time: 0.25 s
    #> Convergence: 3 ( NLOPT_FTOL_REACHED: Optimization stopped because ftol_rel or ftol_abs (above) was reached. )

The same robust variance is available post hoc via `wesml_vcov()` on any fitted mixed logit whose stored data already contain the WESML weights, so you can obtain choice-based-sampling standard errors even from a weighted fit estimated with `se_method = "hessian"` without refitting. `wesml_vcov()` cannot turn an unweighted point estimate into a WESML estimate. More generally, `vcov(fit, type = "robust")` computes the identical sandwich, and `type = "cluster"` extends it to within-cluster dependence — the full menu of variance estimators, and when each is the right choice, is the subject of Which standard errors, and when.

> A note on the multinomial logit. WESML weighting (`weights_col`) and the robust sandwich work the same way on `run_mnlogit()` and `run_nestlogit()` as on the mixed logit shown here. For the plain multinomial logit there is additionally a classical and convenient result (Manski and Lerman, 1977): when the model includes a full set of alternative-specific constants, choice-based sampling leaves the slope coefficients consistently estimated even without weighting — only the ASCs are inconsistent. Each constant is shifted by \(\ln\!\big(H_j / Q_j\big)\) and can be corrected by subtracting that term. So for an MNL with ASCs the substantive marginal-utility parameters are unaffected by the sampling scheme; only the constants (and the predicted shares they drive) need correcting.

## Starting from an existing sample

When the choice-based sample already exists, provide the population shares `Q` directly:

    cb2 <- copy(cb)
    cb2[, .wesml_weight := NULL]

    cb2 <- wesml_weights(
      cb2,
      id_col     = "id",
      alt_col    = "alt",
      choice_col = "choice",
      Q          = attr(cb, "Q"),
      attach     = TRUE
    )

    attr(cb2, "choice_sampling")
    #> $scheme
    #> [1] "wesml"
    #>
    #> $Q
    #>      2      3      1      4
    #> 0.1500 0.3297 0.3550 0.1653
    #>
    #> $H
    #>    1    2    3    4
    #> 0.25 0.25 0.25 0.25
    #>
    #> $meat
    #> [1] "robust"
    #>
    #> $source
    #> [1] "wesml_weights"
    #>
    #> $weight_name
    #> [1] ".wesml_weight"

The names of `Q` must match the chosen-alternative strata exactly after coercion to character. This strict matching is intentional: silently dropping a realized stratum would change the target population.

## The shipped `mode_choice` data

The package’s own `mode_choice` data — the Greene and Hensher intercity study used in the getting-started vignette — is itself a choice-based sample: the survey over-sampled the minority modes and under-sampled car (see `?mode_choice`). It ships without weights because the population mode shares are not part of the data set; they are external information, exactly as `Q` was throughout this vignette. A user with population shares — from transport statistics, or from the textbook treatments of this data set — attaches them in one step and refits:

    data(mode_choice)

    mc <- wesml_weights(
      mode_choice,
      id_col     = "id",
      alt_col    = "mode",
      choice_col = "choice",
      Q          = c(air = Q_air, train = Q_train, bus = Q_bus, car = Q_car),
      attach     = TRUE
    )

    fit_w <- run_mnlogit(
      data           = mc,
      id_col         = "id",
      alt_col        = "mode",
      choice_col     = "choice",
      covariate_cols = c("wait", "travel", "vcost"),
      weights_col    = ".wesml_weight",
      se_method      = "sandwich"
    )

Expect the constants and the fitted shares to move, and the slopes — hence the value-of-time WTP ratios — to stay essentially put: this model carries a full set of alternative-specific constants, so it is exactly the Manski-Lerman special case in the note above.

## References

Manski, C. F. and Lerman, S. R. (1977). The estimation of choice probabilities from choice based samples. Econometrica, 45(8), 1977-1988.

Train, K. E. (2009). Discrete Choice Methods with Simulation (2nd ed.). Cambridge University Press, Section 3.7.