NTPsec

crane3.services.mbix.ca

Report generated: Wed Jul 1 04:45:03 2026 UTC
Start Time: Wed Jun 24 04:45:02 2026 UTC
End Time: Wed Jul 1 04:45:02 2026 UTC
Report Period: 7.0 days

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Local Clock Time/Frequency Offsets

local offset plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local Clock Time Offset -51.536 -23.121 -17.534 -3.226 27.876 39.111 70.332 45.410 62.232 13.851 -0.002 µs -3.067 6.952
Local Clock Frequency Offset 78.412 78.443 78.480 78.663 78.740 78.767 78.807 0.260 0.323 0.081 78.640 ppm 9.274e+08 9.044e+11

The time and frequency offsets between the ntpd calculated time and the local system clock. Showing frequency offset (red, in parts per million, scale on right) and the time offset (blue, in μs, scale on left). Quick changes in time offset will lead to larger frequency offsets.

These are fields 3 (time) and 4 (frequency) from the loopstats log file.



Local RMS Time Jitter

local jitter plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local RMS Time Jitter 6.244 9.221 11.232 18.086 26.356 30.215 42.846 15.124 20.994 4.635 18.368 µs 35.27 137.3

The RMS Jitter of the local clock offset. In other words, how fast the local clock offset is changing.

Lower is better. An ideal system would be a horizontal line at 0μs.

RMS jitter is field 5 in the loopstats log file.



Local RMS Frequency Jitter

local stability plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local RMS Frequency Jitter 2.255 3.474 4.205 6.522 9.154 10.479 16.872 4.949 7.005 1.531 6.588 ppb 46.38 193.5

The RMS Frequency Jitter (aka wander) of the local clock's frequency. In other words, how fast the local clock changes frequency.

Lower is better. An ideal clock would be a horizontal line at 0ppm.

RMS Frequency Jitter is field 6 in the loopstats log file.



Local Clock Time Offset Histogram

local offset histogram plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local Clock Offset -51.536 -23.121 -17.534 -3.226 27.876 39.111 70.332 45.410 62.232 13.851 -0.002 µs -3.067 6.952

The clock offsets of the local clock as a histogram.

The Local Clock Offset is field 3 from the loopstats log file.



Local Temperatures

local temps plot

Local temperatures. These will be site-specific depending upon what temperature sensors you collect data from. Temperature changes affect the local clock crystal frequency and stability. The math of how temperature changes frequency is complex, and also depends on crystal aging. So there is no easy way to correct for it in software. This is the single most important component of frequency drift.

The Local Temperatures are from field 3 from the tempstats log file.



Local Frequency/Temp

local freq temps plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local Clock Frequency Offset 78.412 78.443 78.480 78.663 78.740 78.767 78.807 0.260 0.323 0.081 78.640 ppm 9.274e+08 9.044e+11
Temp LM0 36.000 37.000 38.000 39.000 41.000 41.000 42.000 3.000 4.000 0.946 39.389 °C
Temp LM1 34.000 34.000 35.000 36.000 38.000 39.000 39.000 3.000 5.000 1.072 36.320 °C
Temp LM10 32.000 32.000 33.000 34.000 36.000 37.000 38.000 3.000 5.000 1.063 34.465 °C
Temp LM2 32.000 33.000 33.000 34.000 36.000 37.000 38.000 3.000 4.000 0.961 34.477 °C
Temp LM3 37.000 37.000 38.000 39.000 41.000 41.000 42.000 3.000 4.000 0.941 39.389 °C
Temp LM4 34.000 35.000 35.000 36.000 37.000 38.000 39.000 2.000 3.000 0.811 36.025 °C
Temp LM5 62.000 62.000 62.000 63.000 64.000 64.000 64.000 2.000 2.000 0.636 63.294 °C
Temp LM6 39.000 40.000 40.000 42.000 43.000 43.000 45.000 3.000 3.000 0.964 41.524 °C
Temp LM7 37.000 38.000 39.000 40.000 42.000 42.000 43.000 3.000 4.000 1.048 40.208 °C
Temp LM8 37.000 38.000 39.000 41.000 43.000 43.000 45.000 4.000 5.000 1.096 41.004 °C
Temp LM9 36.000 37.000 38.000 39.000 41.000 42.000 44.000 3.000 5.000 1.101 39.313 °C
Temp ZONE0 39.000 40.000 40.000 42.000 43.000 44.000 44.000 3.000 4.000 1.055 41.663 °C
Temp ZONE1 37.000 37.000 38.000 40.000 41.000 42.000 43.000 3.000 5.000 1.029 39.499 °C

The frequency offsets and temperatures. Showing frequency offset (red, in parts per million, scale on right) and the temperatures.

These are field 4 (frequency) from the loopstats log file, and field 3 from the tempstats log file.



Server Offsets

peer offsets plot

The offset of all refclocks and servers. This can be useful to see if offset changes are happening in a single clock or all clocks together.

Clock Offset is field 5 in the peerstats log file.



Server Offset 132.246.11.227

peer offset 132.246.11.227 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 132.246.11.227 1.555 1.838 1.897 2.098 2.194 17.934 18.080 0.297 16.095 2.372 2.442 ms 6.456 42.39

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset 142.3.100.2

peer offset 142.3.100.2 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 142.3.100.2 -2,543.593 -50.176 -34.669 3.563 45.638 67.817 87.658 80.307 117.993 62.797 2.374 µs -37.09 1481

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset 2600:2600::199 (ntp2.wiktel.com)

peer offset 2600:2600::199 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 2600:2600::199 (ntp2.wiktel.com) -2,772.580 30.583 52.792 366.399 400.719 412.260 429.056 347.927 381.677 112.624 342.211 µs 2.979 233.2

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset 2602:fde5:2a::13

peer offset 2602:fde5:2a::13 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 2602:fde5:2a::13 1.425 1.450 1.460 1.492 2.071 2.086 2.102 0.612 0.636 0.264 1.658 ms 163.9 963.6

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset 2606:4700:f1::1 (time.cloudflare.com)

peer offset 2606:4700:f1::1 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 2606:4700:f1::1 (time.cloudflare.com) -2.143 -1.473 -1.349 -1.162 -0.985 -0.822 -0.370 0.364 0.652 0.119 -1.165 ms -1284 1.42e+04

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset 2607:f388::123:1 (ntp1.doit.wisc.edu)

peer offset 2607:f388::123:1 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 2607:f388::123:1 (ntp1.doit.wisc.edu) -0.589 -0.546 -0.517 -0.457 2.233 4.806 11.829 2.750 5.353 1.103 -0.048 ms -0.5153 13.83

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Offset PPS(0)

peer offset PPS(0) plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset PPS(0) -51.537 -23.122 -17.535 -3.227 27.877 39.112 70.333 45.412 62.234 13.852 -0.002 µs -3.067 6.952

The offset of a server in seconds. This is useful to see how the measured offset is behaving.

The chart also plots offset±rtt, where rtt is the round trip time to the server. NTP can not really know the offset of a remote chimer, NTP computes it by subtracting rtt/2 from the offset. Plotting the offset±rtt reverses this calculation to more easily see the effects of rtt changes.

Closer to 0s is better. An ideal system would be a horizontal line at 0s. Typical 90% ranges may be: local LAN server 80µs; 90% ranges for WAN server may be 4ms and much larger.

Clock Offset is field 5 in the peerstats log file. The Round Trip Time (rtt) is field 6 in the peerstats log file.



Server Jitters

peer jitters plot

The RMS Jitter of all refclocks and servers. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 132.246.11.227

peer jitter 132.246.11.227 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 132.246.11.227 0.024 0.042 0.063 0.263 1.327 4.633 80.072 1.265 4.590 2.752 0.559 ms 20.91 595.7

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 142.3.100.2

peer jitter 142.3.100.2 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 142.3.100.2 0.000 0.016 0.022 0.040 0.095 0.555 182.803 0.073 0.539 8.191 0.562 ms 15.23 324.1

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 2600:2600::199 (ntp2.wiktel.com)

peer jitter 2600:2600::199 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 2600:2600::199 (ntp2.wiktel.com) 4.958 8.151 10.123 19.724 42.329 66.818 4,226.274 32.206 58.667 151.387 29.540 µs 21.15 552.2

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 2602:fde5:2a::13

peer jitter 2602:fde5:2a::13 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 2602:fde5:2a::13 0.004 0.008 0.011 0.038 0.578 0.593 5.015 0.567 0.586 0.292 0.239 ms 5.154 85.36

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 2606:4700:f1::1 (time.cloudflare.com)

peer jitter 2606:4700:f1::1 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 2606:4700:f1::1 (time.cloudflare.com) 5.317 12.668 17.425 44.556 111.730 347.299 1,027.031 94.305 334.631 58.702 55.962 µs 6.7 71.15

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter 2607:f388::123:1 (ntp1.doit.wisc.edu)

peer jitter 2607:f388::123:1 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 2607:f388::123:1 (ntp1.doit.wisc.edu) 0.006 0.010 0.014 0.032 17.754 24.883 54.082 17.740 24.873 6.655 3.246 ms 0.5245 5.608

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Server Jitter PPS(0)

peer jitter PPS(0) plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter PPS(0) 1.695 5.070 7.423 16.970 34.078 43.682 73.377 26.655 38.612 8.275 18.328 µs 6.486 20.81

The RMS Jitter of a server. Jitter is the current estimated dispersion, in other words the variation in offset between samples.

Closer to 0s is better. An ideal system would be a horizontal line at 0s.

RMS Jitter is field 8 in the peerstats log file.



Summary


Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Local Clock Frequency Offset 78.412 78.443 78.480 78.663 78.740 78.767 78.807 0.260 0.323 0.081 78.640 ppm 9.274e+08 9.044e+11
Local Clock Time Offset -51.536 -23.121 -17.534 -3.226 27.876 39.111 70.332 45.410 62.232 13.851 -0.002 µs -3.067 6.952
Local RMS Frequency Jitter 2.255 3.474 4.205 6.522 9.154 10.479 16.872 4.949 7.005 1.531 6.588 ppb 46.38 193.5
Local RMS Time Jitter 6.244 9.221 11.232 18.086 26.356 30.215 42.846 15.124 20.994 4.635 18.368 µs 35.27 137.3
Server Jitter 132.246.11.227 0.024 0.042 0.063 0.263 1.327 4.633 80.072 1.265 4.590 2.752 0.559 ms 20.91 595.7
Server Jitter 142.3.100.2 0.000 0.016 0.022 0.040 0.095 0.555 182.803 0.073 0.539 8.191 0.562 ms 15.23 324.1
Server Jitter 2600:2600::199 (ntp2.wiktel.com) 4.958 8.151 10.123 19.724 42.329 66.818 4,226.274 32.206 58.667 151.387 29.540 µs 21.15 552.2
Server Jitter 2602:fde5:2a::13 0.004 0.008 0.011 0.038 0.578 0.593 5.015 0.567 0.586 0.292 0.239 ms 5.154 85.36
Server Jitter 2606:4700:f1::1 (time.cloudflare.com) 5.317 12.668 17.425 44.556 111.730 347.299 1,027.031 94.305 334.631 58.702 55.962 µs 6.7 71.15
Server Jitter 2607:f388::123:1 (ntp1.doit.wisc.edu) 0.006 0.010 0.014 0.032 17.754 24.883 54.082 17.740 24.873 6.655 3.246 ms 0.5245 5.608
Server Jitter PPS(0) 1.695 5.070 7.423 16.970 34.078 43.682 73.377 26.655 38.612 8.275 18.328 µs 6.486 20.81
Server Offset 132.246.11.227 1.555 1.838 1.897 2.098 2.194 17.934 18.080 0.297 16.095 2.372 2.442 ms 6.456 42.39
Server Offset 142.3.100.2 -2,543.593 -50.176 -34.669 3.563 45.638 67.817 87.658 80.307 117.993 62.797 2.374 µs -37.09 1481
Server Offset 2600:2600::199 (ntp2.wiktel.com) -2,772.580 30.583 52.792 366.399 400.719 412.260 429.056 347.927 381.677 112.624 342.211 µs 2.979 233.2
Server Offset 2602:fde5:2a::13 1.425 1.450 1.460 1.492 2.071 2.086 2.102 0.612 0.636 0.264 1.658 ms 163.9 963.6
Server Offset 2606:4700:f1::1 (time.cloudflare.com) -2.143 -1.473 -1.349 -1.162 -0.985 -0.822 -0.370 0.364 0.652 0.119 -1.165 ms -1284 1.42e+04
Server Offset 2607:f388::123:1 (ntp1.doit.wisc.edu) -0.589 -0.546 -0.517 -0.457 2.233 4.806 11.829 2.750 5.353 1.103 -0.048 ms -0.5153 13.83
Server Offset PPS(0) -51.537 -23.122 -17.535 -3.227 27.877 39.112 70.333 45.412 62.234 13.852 -0.002 µs -3.067 6.952
Temp LM0 36.000 37.000 38.000 39.000 41.000 41.000 42.000 3.000 4.000 0.946 39.389 °C
Temp LM1 34.000 34.000 35.000 36.000 38.000 39.000 39.000 3.000 5.000 1.072 36.320 °C
Temp LM10 32.000 32.000 33.000 34.000 36.000 37.000 38.000 3.000 5.000 1.063 34.465 °C
Temp LM2 32.000 33.000 33.000 34.000 36.000 37.000 38.000 3.000 4.000 0.961 34.477 °C
Temp LM3 37.000 37.000 38.000 39.000 41.000 41.000 42.000 3.000 4.000 0.941 39.389 °C
Temp LM4 34.000 35.000 35.000 36.000 37.000 38.000 39.000 2.000 3.000 0.811 36.025 °C
Temp LM5 62.000 62.000 62.000 63.000 64.000 64.000 64.000 2.000 2.000 0.636 63.294 °C
Temp LM6 39.000 40.000 40.000 42.000 43.000 43.000 45.000 3.000 3.000 0.964 41.524 °C
Temp LM7 37.000 38.000 39.000 40.000 42.000 42.000 43.000 3.000 4.000 1.048 40.208 °C
Temp LM8 37.000 38.000 39.000 41.000 43.000 43.000 45.000 4.000 5.000 1.096 41.004 °C
Temp LM9 36.000 37.000 38.000 39.000 41.000 42.000 44.000 3.000 5.000 1.101 39.313 °C
Temp ZONE0 39.000 40.000 40.000 42.000 43.000 44.000 44.000 3.000 4.000 1.055 41.663 °C
Temp ZONE1 37.000 37.000 38.000 40.000 41.000 42.000 43.000 3.000 5.000 1.029 39.499 °C
Summary as CSV file


Glossary:

frequency offset:
The difference between the ntpd calculated frequency and the local system clock frequency (usually in parts per million, ppm)
jitter, dispersion:
The short term change in a value. NTP measures Local Time Jitter, Refclock Jitter, and Server Jitter in seconds. Local Frequency Jitter is in ppm or ppb.
kurtosis, Kurt:
The kurtosis of a random variable X is the fourth standardized moment and is a dimension-less ratio. ntpviz uses the Pearson's moment coefficient of kurtosis. A normal distribution has a kurtosis of three. NIST describes a kurtosis over three as "heavy tailed" and one under three as "light tailed".
ms, millisecond:
One thousandth of a second = 0.001 seconds, 1e-3 seconds
mu, mean:
The arithmetic mean: the sum of all the values divided by the number of values. The formula for mu is: "mu = (∑xi) / N". Where xi denotes the data points and N is the number of data points.
ns, nanosecond:
One billionth of a second, also one thousandth of a microsecond, 0.000000001 seconds and 1e-9 seconds.
percentile:
The value below which a given percentage of values fall.
ppb, parts per billion:
Ratio between two values. These following are all the same: 1 ppb, one in one billion, 1/1,000,000,000, 0.000,000,001, 1e-9 and 0.000,000,1%
ppm, parts per million:
Ratio between two values. These following are all the same: 1 ppm, one in one million, 1/1,000,000, 0.000,001, and 0.000,1%
‰, parts per thousand:
Ratio between two values. These following are all the same: 1 ‰. one in one thousand, 1/1,000, 0.001, and 0.1%
refclock:
Reference clock, a local GPS module or other local source of time.
remote clock:
Any clock reached over the network, LAN or WAN. Also called a peer or server.
time offset:
The difference between the ntpd calculated time and the local system clock's time. Also called phase offset.
σ, sigma:
Sigma denotes the standard deviation (SD) and is centered on the arithmetic mean of the data set. The SD is simply the square root of the variance of the data set. Two sigma is simply twice the standard deviation. Three sigma is three times sigma. Smaller is better.
The formula for sigma is: "σ = √[ ∑(xi-mu)^2 / N ]". Where xi denotes the data points and N is the number of data points.
skewness, Skew:
The skewness of a random variable X is the third standardized moment and is a dimension-less ratio. ntpviz uses the Pearson's moment coefficient of skewness. Wikipedia describes it best: "The qualitative interpretation of the skew is complicated and unintuitive."
A normal distribution has a skewness of zero.
upstream clock:
Any server or reference clock used as a source of time.
µs, us, microsecond:
One millionth of a second, also one thousandth of a millisecond, 0.000,001 seconds, and 1e-6 seconds.



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