NTPsec

crane3.services.mbix.ca

Report generated: Mon May 4 16:45:03 2026 UTC
Start Time: Mon Apr 27 16:45:02 2026 UTC
End Time: Mon May 4 16:45:02 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 -46.988 -25.603 -19.250 -3.769 30.571 41.924 71.635 49.821 67.527 15.225 0.000 µs -3.107 6.894
Local Clock Frequency Offset 78.095 78.178 78.233 78.395 78.539 78.575 78.655 0.305 0.398 0.094 78.395 ppm 5.846e+08 4.888e+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 5.876 9.806 12.306 20.012 28.937 33.035 44.393 16.631 23.229 5.087 20.239 µs 35.67 138.6

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 2.171 3.727 4.569 7.193 10.061 11.431 17.132 5.492 7.704 1.676 7.243 ppb 46.86 194.8

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 -46.988 -25.603 -19.250 -3.769 30.571 41.924 71.635 49.821 67.527 15.225 0.000 µs -3.107 6.894

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 78.095 78.178 78.233 78.395 78.539 78.575 78.655 0.305 0.398 0.094 78.395 ppm 5.846e+08 4.888e+11
Temp LM0 35.000 35.000 36.000 37.000 39.000 39.000 41.000 3.000 4.000 0.894 37.115 °C
Temp LM1 31.000 33.000 33.000 35.000 37.000 37.000 38.000 4.000 4.000 1.020 34.943 °C
Temp LM10 32.000 33.000 34.000 35.000 37.000 38.000 39.000 3.000 5.000 1.160 35.150 °C
Temp LM2 30.000 31.000 32.000 33.000 35.000 36.000 36.000 3.000 5.000 0.973 33.319 °C
Temp LM3 35.000 35.000 36.000 37.000 39.000 39.000 40.000 3.000 4.000 0.914 37.140 °C
Temp LM4 33.000 34.000 34.000 35.000 36.000 37.000 38.000 2.000 3.000 0.775 35.239 °C
Temp LM5 61.000 61.000 62.000 63.000 63.000 64.000 64.000 1.000 3.000 0.552 62.633 °C
Temp LM6 40.000 40.000 41.000 42.000 43.000 43.000 45.000 2.000 3.000 0.672 41.849 °C
Temp LM7 37.000 38.000 39.000 40.000 41.000 42.000 44.000 2.000 4.000 0.827 40.196 °C
Temp LM8 38.000 39.000 40.000 41.000 42.000 43.000 45.000 2.000 4.000 0.789 41.017 °C
Temp LM9 37.000 38.000 39.000 40.000 41.000 42.000 43.000 2.000 4.000 0.761 39.712 °C
Temp ZONE0 40.000 40.000 41.000 42.000 43.000 43.000 45.000 2.000 3.000 0.728 41.938 °C
Temp ZONE1 35.000 36.000 36.000 37.000 39.000 40.000 41.000 3.000 4.000 1.068 37.406 °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 1.413 1.834 1.882 2.004 2.171 2.218 6.804 0.289 0.384 0.328 2.038 ms 170.4 1359

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 -166.688 -0.063 -0.050 -0.011 0.039 0.063 20.023 0.089 0.126 3.958 -0.114 ms -41.76 1704

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) 306.563 323.275 335.415 368.761 399.693 411.243 437.458 64.278 87.968 19.696 368.038 µs 5585 9.972e+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 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 1.426 1.446 1.460 1.504 2.131 2.146 2.175 0.671 0.700 0.300 1.716 ms 120 642.5

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::123 (time.cloudflare.com)

peer offset 2606:4700:f1::123 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Offset 2606:4700:f1::123 (time.cloudflare.com) -2.712 -2.419 -2.256 -1.996 -1.687 -1.439 0.192 0.569 0.980 0.242 -1.973 ms -793.2 7454

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) -715.895 -540.658 -509.508 -461.747 -392.586 -254.933 -154.377 116.922 285.725 47.092 -457.391 µs -1260 1.382e+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 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) -46.989 -25.604 -19.251 -3.770 30.572 41.925 71.636 49.823 67.529 15.225 0.000 µs -3.107 6.894

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 0.026 0.047 0.072 0.326 1.714 4.529 144.803 1.641 4.482 4.704 0.715 ms 25.77 788.6

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 0.008 0.016 0.021 0.039 0.092 0.952 180.203 0.072 0.936 6.344 0.447 ms 17.12 415.6

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) 5.531 8.848 11.509 23.142 44.982 56.531 87.466 33.473 47.683 10.765 25.322 µs 7.665 25.57

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 4.342 8.851 12.872 30.465 621.899 648.004 4,847.309 609.027 639.153 300.741 201.767 µs 4.06 59.77

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::123 (time.cloudflare.com)

peer jitter 2606:4700:f1::123 plot

Percentiles...... Ranges...... Skew- Kurt-
Name Min1%5%50%95% 99%Max   90%98%StdDev  MeanUnits nessosis
Server Jitter 2606:4700:f1::123 (time.cloudflare.com) 5.872 11.087 15.777 36.003 122.452 331.870 2,456.882 106.675 320.783 99.438 53.490 µs 14.48 306.3

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) 5.913 9.396 13.149 25.506 112.990 426.276 863.916 99.841 416.880 69.045 41.134 µs 4.587 35.25

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) 1.608 5.281 7.865 18.771 37.396 47.139 77.310 29.531 41.858 9.051 20.163 µs 6.484 20.3

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 78.095 78.178 78.233 78.395 78.539 78.575 78.655 0.305 0.398 0.094 78.395 ppm 5.846e+08 4.888e+11
Local Clock Time Offset -46.988 -25.603 -19.250 -3.769 30.571 41.924 71.635 49.821 67.527 15.225 0.000 µs -3.107 6.894
Local RMS Frequency Jitter 2.171 3.727 4.569 7.193 10.061 11.431 17.132 5.492 7.704 1.676 7.243 ppb 46.86 194.8
Local RMS Time Jitter 5.876 9.806 12.306 20.012 28.937 33.035 44.393 16.631 23.229 5.087 20.239 µs 35.67 138.6
Server Jitter 132.246.11.227 0.026 0.047 0.072 0.326 1.714 4.529 144.803 1.641 4.482 4.704 0.715 ms 25.77 788.6
Server Jitter 142.3.100.2 0.008 0.016 0.021 0.039 0.092 0.952 180.203 0.072 0.936 6.344 0.447 ms 17.12 415.6
Server Jitter 2600:2600::199 (ntp2.wiktel.com) 5.531 8.848 11.509 23.142 44.982 56.531 87.466 33.473 47.683 10.765 25.322 µs 7.665 25.57
Server Jitter 2602:fde5:2a::13 4.342 8.851 12.872 30.465 621.899 648.004 4,847.309 609.027 639.153 300.741 201.767 µs 4.06 59.77
Server Jitter 2606:4700:f1::123 (time.cloudflare.com) 5.872 11.087 15.777 36.003 122.452 331.870 2,456.882 106.675 320.783 99.438 53.490 µs 14.48 306.3
Server Jitter 2607:f388::123:1 (ntp1.doit.wisc.edu) 5.913 9.396 13.149 25.506 112.990 426.276 863.916 99.841 416.880 69.045 41.134 µs 4.587 35.25
Server Jitter PPS(0) 1.608 5.281 7.865 18.771 37.396 47.139 77.310 29.531 41.858 9.051 20.163 µs 6.484 20.3
Server Offset 132.246.11.227 1.413 1.834 1.882 2.004 2.171 2.218 6.804 0.289 0.384 0.328 2.038 ms 170.4 1359
Server Offset 142.3.100.2 -166.688 -0.063 -0.050 -0.011 0.039 0.063 20.023 0.089 0.126 3.958 -0.114 ms -41.76 1704
Server Offset 2600:2600::199 (ntp2.wiktel.com) 306.563 323.275 335.415 368.761 399.693 411.243 437.458 64.278 87.968 19.696 368.038 µs 5585 9.972e+04
Server Offset 2602:fde5:2a::13 1.426 1.446 1.460 1.504 2.131 2.146 2.175 0.671 0.700 0.300 1.716 ms 120 642.5
Server Offset 2606:4700:f1::123 (time.cloudflare.com) -2.712 -2.419 -2.256 -1.996 -1.687 -1.439 0.192 0.569 0.980 0.242 -1.973 ms -793.2 7454
Server Offset 2607:f388::123:1 (ntp1.doit.wisc.edu) -715.895 -540.658 -509.508 -461.747 -392.586 -254.933 -154.377 116.922 285.725 47.092 -457.391 µs -1260 1.382e+04
Server Offset PPS(0) -46.989 -25.604 -19.251 -3.770 30.572 41.925 71.636 49.823 67.529 15.225 0.000 µs -3.107 6.894
Temp LM0 35.000 35.000 36.000 37.000 39.000 39.000 41.000 3.000 4.000 0.894 37.115 °C
Temp LM1 31.000 33.000 33.000 35.000 37.000 37.000 38.000 4.000 4.000 1.020 34.943 °C
Temp LM10 32.000 33.000 34.000 35.000 37.000 38.000 39.000 3.000 5.000 1.160 35.150 °C
Temp LM2 30.000 31.000 32.000 33.000 35.000 36.000 36.000 3.000 5.000 0.973 33.319 °C
Temp LM3 35.000 35.000 36.000 37.000 39.000 39.000 40.000 3.000 4.000 0.914 37.140 °C
Temp LM4 33.000 34.000 34.000 35.000 36.000 37.000 38.000 2.000 3.000 0.775 35.239 °C
Temp LM5 61.000 61.000 62.000 63.000 63.000 64.000 64.000 1.000 3.000 0.552 62.633 °C
Temp LM6 40.000 40.000 41.000 42.000 43.000 43.000 45.000 2.000 3.000 0.672 41.849 °C
Temp LM7 37.000 38.000 39.000 40.000 41.000 42.000 44.000 2.000 4.000 0.827 40.196 °C
Temp LM8 38.000 39.000 40.000 41.000 42.000 43.000 45.000 2.000 4.000 0.789 41.017 °C
Temp LM9 37.000 38.000 39.000 40.000 41.000 42.000 43.000 2.000 4.000 0.761 39.712 °C
Temp ZONE0 40.000 40.000 41.000 42.000 43.000 43.000 45.000 2.000 3.000 0.728 41.938 °C
Temp ZONE1 35.000 36.000 36.000 37.000 39.000 40.000 41.000 3.000 4.000 1.068 37.406 °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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