# Control Group

**Category:** metrics  
**Short Description:** A segment of users or data points that receive no treatment or intervention, serving as a baseline for comparison in experiments.  
**Last Updated:** 2025-03-12T00:00:00Z

## Definition

A control group is a randomly selected segment of users or data points that receive no experimental treatment, serving as the baseline against which test groups are measured. In marketing experimentation, control groups enable marketers to isolate the true causal impact of campaigns, creative changes, or other interventions by comparing outcomes between exposed and unexposed audiences under otherwise identical conditions.

## Calculation

**Formula:** `Lift = (Test - Control) / Control`

**Explanation:** Calculates the relative performance difference between test and control groups. Positive values indicate the treatment had a beneficial effect, while negative values suggest the treatment underperformed the control.

### Components

- **Test Group Metric**: Performance metric from the treatment group
- **Control Group Metric**: Performance metric from the control group

## Examples

- Withholding 10% of users from a campaign to measure true incremental impact
- Using geographic holdouts to measure regional campaign effectiveness
- Implementing PSA (Public Service Announcement) tests as active controls
- Creating persistent holdout groups for long-term incrementality measurement

## Best Practices

- Ensure sufficient sample size for statistical validity
- Use true randomization for group assignment
- Maintain clean separation between test and control groups
- Document all external factors that might influence results
- Consider using multiple control group types for robust validation

## Related Terms

### Similar Terms

- **[A/B Testing](/resources/glossary/creative/ab-testing)**: Experimental methodology requiring control groups
- **[Incrementality](/resources/glossary/general/incrementality)**: Measurement approach using control groups to isolate true impact

### Component Terms

- **[Statistical Significance](/resources/glossary/metrics/statistical-significance)**: Determines reliability of test vs. control comparisons

## Featured in topic hubs

- [Experimentation & Statistics](/resources/topics/experimentation)