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Percent-change metric alert reports unrealistic deviation #113912

@dalnoki

Description

@dalnoki

Environment

SaaS (https://sentry.io/)

Steps to Reproduce

Percent-change metric alerts are reporting wildly inaccurate deviation values. With a maximum deviation threshold set to 20%, alerts are firing and reporting values like 113% higher compared to the same time one day ago.

However, inspecting the p90(span.duration) chart over the full 24-hour window shows a stable, consistent signal between ~3000–4000ms with no gaps, zero values, or anomalous drops. The dashed comparison baseline tracks nearly identically to the current period. There is no point in the visible data where a ~113% deviation could be computed from the two windows being compared.

Expected Result

The percent change reported in the alert notification matches the actual deviation visible in the span duration chart for the same time windows.

Actual Result

The alert fires reporting a 113% increase in p90(span.duration) over the last 30 minutes compared to the same time one day ago.

Product Area

Alerts

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