### Summary

This metric reports the middle amplitude value in counts of sorted amplitude values from a 24-hour window. This median is one measure of the central tendency of the amplitudes in a time series when values are arranged in sorted order. The median value itself always occurs as an amplitude value in the times series.

### Uses

Like the mean, the median amplitude of a time series describes its static behavior. For a properly functioning instrument, it can echo a channel’s DC level and/or mass position. Sudden changes in median can indicate partial loss of differential output, particularly if the RMS variance suddenly decreases simultaneously. The median is less sensitive to outliers than the mean.

### Data Analyzed

**Traces** – one N.S.L.C (Network.Station.Location.Channel) per measurement

**Window** – 24 hours starting at 00:00:00 UTC

**Data Source** – IRIS miniSEED archive or IRIS PH5 archive

**SEED Channel Types** –
?H?, ?L?, ?N?, ?G?, ?P?, VM? | High Gain, Low Gain, Accelerometer, Gravimeter, Geophone, Mass Channels

### Algorithm

- Request 24 hours of data for a single channel.
- Identify and report the median amplitude value for the requested data:

For i=1,N samples in the time series x: y(i) = sort{x(i)} median = y(N/2)

### Metric Values Returned

**value** – median amplitude in counts

**target** – the trace analyzed, labeled as N.S.L.C.Q (Network.Station.Location.Channel.Quality)

**start** – beginning of the data day requested (00:00:00 UTC)

**end** – end of the data day requested (truncated as 23:59:59 UTC)

**lddate** – date/time the measurement was made and loaded into the MUSTANG database (UTC)

### Notes

Instrument response has not been removed.