ssepy: A Library for Efficient Model Evaluation through Stratification, Sampling, and Estimation in Python

Given an unlabeled dataset and model predictions, how can we select which instances to annotate in one go to maximize the precision of our estimates of model performance on the entire dataset?

ssepy helps you estimate the mean of any random variable across a large dataset. When the focus is on a model’s performance, it treats each sample’s performance as a random variable and aims to estimate the average (i.e., mean) performance over the entire dataset.

The main idea:

1. Predict: Obtain a proxy or predicted value for each sample (e.g., a model’s predicted performance on that sample).
2. Stratify: Use these proxies to group the samples into strata.
3. Sample: From each stratum, draw a subset of samples according to the chosen allocation method (proportional, Neyman, or others).
4. Annotate: Acquire ground-truth labels or real outcomes for the sampled subset.
5. Estimate: Compute the overall mean (e.g., the mean model performance) using an estimator such as Horvitz-Thompson or a difference estimator.

See our paper here for a technical overview of the framework.

Getting started

In order to intall the package, run

pip install ssepy

Alternatively, clone the repo, cd into it, and run

pip install .

You may want to initialize a conda environment before running this operation.

Test your setup using this example, which demonstrates data stratification, n allocation for annotation via proportional allocation, sampling via stratified simple random sampling, and estimation using the Horvitz-Thompson estimator:

import numpy as np
from sklearn.cluster import KMeans
from ssepy import ModelPerformanceEvaluator

np.random.seed(0)
# Generate data
N = 100000
Y = np.random.normal(0, 1, N) # Ground truth

# Unobserved target
print(np.mean(Y))

n = 100 # Annotation n
# 1. Proxy for ground truth
Yh = Y + np.random.normal(0, 0.1, N)
evaluator = ModelPerformanceEvaluator(Yh = Yh, budget = n) # Initialize evaluator
# 2. Stratify on Yh
evaluator.stratify_data(clustering_algo=KMeans(n_clusters=5, random_state=0, n_init="auto"), X=Yh) # 5 strata
# 3. Allocate n with proportional allocation and sample
evaluator.allocate_budget(allocation_type="proportional")
sampled_idx = evaluator.sample()
# 4. Annotate
Yl = Y[sampled_idx]
# 5. Estimate target and variance of estimate
estimate, variance_estimate = evaluator.compute_estimate(Yl, estimator="ht")
print(estimate, variance_estimate)

For the difference estimator under simple random sampling, run

evaluator = ModelPerformanceEvaluator(Yh=Yh, budget=n) # initialize sampler
sampled_idx = evaluator.sample(sampling_method="srs") # 3. sample
Yl = Y[sampled_idx] # 4. annotate
estimate, variance_estimate = evaluator.compute_estimate(Yl, estimator="df") # 5. estimate
print(estimate, variance_estimate)

See also some examples in the associated folder.

Features

The supported sample designs are: (SRS) simple random sampling without replacement, (SSRS) stratified simple random sampling without replacement with proportional and optimal/Neyman allocation, (Poisson) sampling. All sampling methods have associated (HT) Horvitz-Thompson and (DF) difference estimators.

Bugs and contribute

Feel free to reach out if you find any bugs or you would like other features to be implemented in the package.