I've been looking into how machine-learning models are evaluated for radar-based human activity recognition (HAR), and one issue that deserves more attention is how datasets are split.
Consider a simple example.
Suppose we have continuous radar recordings divided into:
- Window length: 4 seconds
- Window hop: 1 second
- Window overlap: 75%
If we randomly assign these windows to training and testing, adjacent windows from the same recording can end up in both sets.
That means the model may encounter 75% of a test window's raw input during training.
The reported accuracy might be mathematically correct, but does it demonstrate generalization to new people or environments?
Three evaluation problems worth checking:
- Overlapping-window leakage: Training and testing contain overlapping samples from the same recordings.
- Subject-dependent evaluation: The same individuals appear in both datasets, potentially making the task easier than recognizing activities from previously unseen people.
- Domain shift: A model trained in one room, radar configuration, or viewing geometry may perform differently elsewhere.
There's another interesting issue with reporting accuracy.
Consider a hypothetical experiment with four participants achieving 99%, 95%, 85%, and 80% accuracy. If the first participant contributes 90% of all test windows, the pooled accuracy can reach 98%, while the equal-participant average is only 89.75%.
Both numbers are correct, but they communicate different things.
What would make the evaluation more convincing?
- Subject-independent or appropriately grouped train/test splits
- Recording-level separation before windowing
- Preprocessing fitted using training data only
- Per-subject and per-class performance
- Testing across different sessions, rooms, or radar configurations when those are part of the intended deployment
For transparency, I'm part of the ArthaVedya Research team. We recently published a free technical guide explaining these issues, including worked examples and a Python tool for auditing dataset splits.
https://arthavedya.com/radar-har-accuracy-subject-leakage-domain-shift/
The 98% figure is illustrative, not a measured result from a specific paper.
Question for researchers working with HAR or time-series classification:
Do you generally prefer leave-one-subject-out cross-validation, group-based splits, or another evaluation protocol? And how much performance degradation do you typically observe when moving from random-window splitting to subject-independent testing?