# 4.1 Observed standard deviation is too low, ........
BB2BUG-17449 - \[BB2\]\[RTL8763EF/ROM\]\[GMI\]\[沃莱特\]\[Harman\]\[T520\]: Finder测项中，finder_advertisement rotation测试失败 Closed
Hi Avner:
We test advertising rotation failed, could you help see the reason, thanks
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Hi Danni,\
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The reason that this test fails is that the device under test rotates EID/MAC pretty much precisely every 1024 seconds (standard deviation is 0.83 seconds).\
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Per spec the exact rotation time should be randomized, please refer to the "ID Rotation" section.
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Hi Mark:
1.Does the positive randomized time factor have a value range? Can you provide some reference ranges?
2.Is the standard deviation must be greater than 0.83?
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+David Lazarov Can you please attach the latest version of the spec which includes the recommended randomization values?
Hi Danni,
1. The recommended size of the randomization window is 20% of the interval, i.e. 204 seconds. So when the device needs to compute the next time point to rotate EID/MAC, it's going to be *floor((last_rotation_time_seconds + 1024) / 1024) \* 1024 + random(0, 204). *
2. *Standard deviation of 0.83 seconds practically means there's no randomization. We expect standard deviation to be much higher, typically 50-60 seconds, or more.*
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