How to measure segment performance over time?

Segment testing validates whether your segmentation actually improves performance over sending to everyone.

Baseline comparison:

Compare segmented campaign performance to unsegmented
Measure: open rate, click rate, conversion rate, unsubscribe rate, complaint rate
If segmented doesn't outperform, the segmentation isn't adding value

Segment vs segment comparison:

Send the same content to different segments
Do they respond differently as expected?
If "hiking enthusiasts" don't respond better to hiking content, your segment definition may be wrong

Holdout testing:

Within a segment, create a holdout group that gets generic content
Compare: Does the tailored content outperform generic?
Measures whether personalization for that segment works

Metrics to evaluate:

Engagement: Do segments have meaningfully different response rates?
Conversion: Do targeted offers convert better than generic?
Retention: Do segmented subscribers unsubscribe less?
ROI: Does the effort of segmentation pay off financially?

Signs of effective segmentation:

Clear performance differences between segments
Higher overall engagement than unsegmented approach
Lower complaint and unsubscribe rates
Better conversion and revenue metrics

Signs segmentation isn't working:

Segments perform similarly to each other
No improvement over unsegmented sends
Complexity without corresponding lift

A compass that doesn't help you navigate is just a decoration. Test whether your segments actually guide you to better results.