What are populations?
Understand population models
Populations are the core of primitive of the Semilattice API.
What is a population?
A population is a model that represents a specific group of people in reality. When you call the API with a population_id
, you’re telling Semilattice which group to simulate.
Target population
Population models have a reality_target
metadata field which describes the group of people in reality the model is intended to represent. When choosing a population model, select the one that best matches your target audience or user profile.

A Population modelling global, professional devops specialists
Populations range from broad user profiles to highly specific niches.
Seed data
Populations are built using tabular question answer data (such as survey data) from the target audience. This seed data determines:
- Prediction accuracy: How well the model predicts the target audience
- Subject expertise: What topics the population can predict accurately

Seed data define population models
Learn more about how populations are created in the seed data section.
Accuracy evaluation
Every population model, once tested, has evaluation metrics that help you understand prediction accuracy:

Accuracy metrics help assess prediction confidence
These metrics appear in API responses and help you:
- Choose the right population for your use case
- Set appropriate confidence levels for your predictions
- Understand the reliability of specific results
Learn more about these metrics and how to interpret them in the Accuracy Evaluation guide.
Simulation engine
Each population uses a specific version of Semilattice’s simulation engine. This ensures:
- Consistency: All predictions use the same model version
- Accuracy: Evaluation metrics reflect real prediction performance
- Reliability: Results are reproducible

Populations use consistent simulation engines
The simulation_engine
field in API responses tells you which engine was used. All public population models use the answers-1
simulation engine.
Public vs. custom populations
When making API calls, you can access:
- Public populations: Available to all users with valid API keys
- Custom populations: Only accessible to the owner’s API key
Public populations
Public populations
- Can be used by any authenticated API user
- Ideal for common user profiles (“US Adults”, “Developers”)
- IDs can be shared in documentation and examples
Custom populations
Custom populations
- Only accessible with the owner’s API key
- Perfect for proprietary customer segments
- IDs should be kept secure like API keys, though only owners of a custom population can use it for simulations

API access depends on population privacy settings
Finding population IDs
To use a population in your API calls, you need its ID. Navigate to the Populations page in your dashboard and select a population model to use. Copy the ID from the population’s metadata or from the address bar.

Click to copy the population model's ID