Drift and Drift Correction

What is drift? 

”Drift” refers to the amount by which the count of a space in our system differs from the actual number of people in the space. There are two types of drift:

• Positive Drift: Occurs when exits are missed or extra entrances are counted by a sensor causing positive counts
• Negative Drift: Occurs when entrances are missed or extra exits are counted by a sensor causing negative counts

What is drift correction?

We normalize the data across the day, from the assumption that the count should be 0 at the daily reset time. When you query data, we adjust the slope of either the exits (in the case of positive drift) or the entrances (in the case of negative drift). This results in the same trend over the day, but without the steep drop-off at the reset time.  

Before:

Visible drop in occupancy at the 4 a.m. reset time, with the count staying at the same high number from 6 p.m.

After: 

Data is smoothed out naturally across the hours, taking into account that the count should be 0 at 4 a.m., but occupancy did not significantly change after 6 p.m.

Drift Correction Changes

1. Drift correction is applied at query time. Changing the daily reset time will change the results for all historical data. You can adjust the daily reset time multiple times to find the setting that makes the most sense for your space by emailing support@density.io.
2. Daily resets should be set for when your space is completely empty, including cleaning crews and night staff.
3. The daily reset time default is set to 00:00. You can adjust this at any time.
4. Drift correction is only applied after the daily reset time. Please wait 24 hours to look at the data for Entry.

See also:

Understanding Drift in Entry-Based Occupancy Counting