# 12 Pathfinder Method for Approximate Bayesian Inference

The Pathfinder algorithm is described in section Pathfinder overview.

The pathfinder method runs multi-path Pathfinder by default, which returns a PSIS sample over the draws from several individual (“single-path”) Pathfinder runs. Argument num_paths specifies the number of single-path Pathfinders, the default is $$4$$. If num_paths is set to 1, then only one individual Pathfinder is run without the PSIS reweighting of the sample.

The full set of configuration options available for the pathfinder method is reported at the beginning of the pathfinder output file as CSV comments. When the example model bernoulli.stan is run with method=pathfinder via the command line with all default arguments, the resulting Stan CSV file header comments show the complete set of default configuration options:

method = pathfinder
pathfinder
init_alpha = 0.001 (Default)
tol_obj = 9.9999999999999998e-13 (Default)
tol_rel_obj = 10000 (Default)
tol_grad = 1e-08 (Default)
tol_rel_grad = 10000000 (Default)
tol_param = 1e-08 (Default)
history_size = 5 (Default)
num_psis_draws = 1000 (Default)
num_paths = 4 (Default)
save_single_paths = 0 (Default)
max_lbfgs_iters = 1000 (Default)
num_draws = 1000 (Default)
num_elbo_draws = 25 (Default)

## 12.1 Pathfinder Configuration

• num_psis_draws - Final number of draws from multi-path pathfinder. Must be a positive integer. Default value is $$1000$$.

• num_paths - Number of single pathfinders. Must be a positive integer. Default value is $$4$$.

• save_single_paths - When True ($$1$$), save outputs from single pathfinders. Valid values: $$\{0, 1\}$$. Default is $$0$$ (False).

• max_lbfgs_iters - Maximum number of L-BFGS iterations. Must be a positive integer. Default value is $$1000$$.

• num_draws - Number of approximate posterior draws for each single pathfinder. Must be a positive integer. Default value is $$1000$$. Can differ from num_psis_draws.

• num_elbo_draws - Number of Monte Carlo draws to evaluate ELBO. Must be a positive integer. Default value is $$25$$.

## 12.2 L-BFGS Configuration

Arguments init_alpha through history_size are the full set of arguments to the L-BFGS optimizer and have the same defaults for optimization.

## 12.3 Multi-path Pathfinder CSV files

By default, the pathfinder method uses 4 independent Pathfinder runs, each of which produces 1000 approximate draws, which are then importance resampled down to 1000 final draws. The importance resampled draws are output as a StanCSV file file.

The CSV files have the following structure:

• The full set of configuration options available for the pathfinder method is reported at the beginning of the sampler output file as CSV comments.

• The CSV header row consists of columns lp_approx__, lp__, and the Stan model parameters, transformed parameters, and generated quantities in the order in which they are declared in the Stan program.

• The data rows contain the draws from the single- or multi-path run.

• Final comments containing timing information.

## 12.4 Single-path Pathfinder Outputs.

The boolean option save_single_paths is used to save both the draws and the ELBO iterations from the individual Pathfinder runs. When save_single_paths is $$1$$, the draws from each are saved to StanCSV files with the same format as the PSIS sample and the ELBO evaluations along the L-BFGS trajectory for each are saved as JSON. Given an output file name, CmdStan adds suffixes to the base filename to distinguish between the output files. For the default output file name output.csv and default number of runs (4), the resulting CSV files are

output.csv
output_path_1.csv
output_path_1.json
output_path_2.csv
output_path_2.json
output_path_3.csv
output_path_3.json
output_path_4.csv
output_path_4.json

The individual sample CSV files have the same structure as the PSIS sample CSV file. The JSON files contain information from each ELBO iteration.

To see how this works, we run Pathfinder on the centered-parameterization of the eight-schools model, where the posterior distribution has a funnel shape:

> eight_schools pathfinder save_single_paths=1 data file=eight_schools.data.json

Each JSON file records the approximations to the target density at each point along the trajectory of the L-BFGS optimization algorithms.

{
"0": {
"iter": 0,
"unconstrained_parameters": [1.00595, -0.503687, 1.79367, 0.99083, 0.498077, -0.65816, 1.49176, -1.22647, 1.62911, 0.767445],
"grads": [-0.868919, 0.45198, -0.107675, -0.0123304, 0.163172, 0.354362, -0.108746, 0.673306, -0.102268, -4.51445]
},
"1": {
"iter": 1,
"unconstrained_parameters": [1.00595, -0.503687, 1.79367, 0.99083, 0.498077, -0.65816, 1.49176, -1.22647, 1.62911, 0.767445],
"grads": [-0.868919, 0.45198, -0.107675, -0.0123304, 0.163172, 0.354362, -0.108746, 0.673306, -0.102268, -4.51445],
"history_size": 1,
"lbfgs_success": true,
"pathfinder_success": true,
"x_center": [0.126047, -0.065048, 1.55708, 0.958509, 0.628075, -0.217041, 1.32032, -0.561338, 1.42988, 1.23213],
"logDetCholHk": -2.6839,
"L_approx": [[-0.0630456, -0.0187959], [0, 1.08328]],
"Qk": [[-0.361073, 0.5624], [0.183922, -0.279474], [-0.0708175, 0.15715], [-0.00917823, 0.0215802], [0.0606019, -0.0814513], [0.164071, -0.285769], [-0.057723, 0.112428], [0.276376, -0.424348], [-0.0620524, 0.131786], [-0.846488, -0.531094]],
"alpha": [1, 1, 1, 1, 1, 1, 1, 1, 1, 1],
"full": false,
"lbfgs_note": ""
},
...,
"171": {
"iter": 171,
"unconstrained_parameters": [1.60479, 1.60479, 1.60479, 1.60479, 1.60479, 1.60479, 1.60479, 1.60479, 1.60479, -35.7821],
"grads": [2.66927e+15, -0.117312, -0.0639521, -2.66927e+15, -0.0445885, 0.0321579, 0.00499827, -0.163952, -0.032084, 6.4073],
"history_size": 5,
"lbfgs_success": true,
"pathfinder_success": true,
"x_center": [5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, 5.58876e+15, -2.02979e+17],
"logDetCholHk": 299.023,
"L_approx": [[4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, 4.6852e+06, -1.70162e+08], [0, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 0, 2.19511e+13, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 0, 0, 2.19511e+13, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 0, 0, 0, 2.19511e+13, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 0, 0, 0, 0, 2.19511e+13, -7.97244e+14], [0, 0, 0, 0, 0, 0, 0, 0, 0, 2.89552e+16]],
"Qk": [],
"alpha": [1.11027e-12, 2.24669e-12, 2.05603e-12, 3.71177e-12, 5.7855e-12, 1.80169e-12, 3.40291e-12, 2.29699e-12, 3.43423e-12, 1.25815e-08],
"full": true,
"lbfgs_note": ""
},
"172": {
"iter": 172,
"unconstrained_parameters": [1.60531, 1.60531, 1.60531, 1.60531, 1.60531, 1.60531, 1.60531, 1.60531, 1.60531, -35.801],
"grads": [-0, -0.11731, -0.0639469, 0.0179895, -0.0445842, 0.0321643, 0.00500256, -0.163947, -0.0320824, 7],
"history_size": 5,
"lbfgs_success": false,
"pathfinder_success": false,
"lbfgs_note": ""
}
}

Option num_paths=1 runs one single-path Pathfinder and output CSV file contains the draws from that run without PSIS reweighting. The combination of arguments num_paths=1 save_single_paths=1 creates just two output files, the CSV sample and the set of ELBO iterations. In this case, the default output file name is “output.csv” and the default diagnostic file name is “output.json”.