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Reading the SPC chart

tsdive spc runs three SPC rules over a window, with limits from a baseline. This is the report on the demo flow, then every line.

$ tsdive spc data/demo/fic101_demo.parquet \
    --baseline 2024-03-30T20:00:00Z/2024-03-31T01:00:00Z \
    --window 2024-03-31T01:00:00Z/2024-03-31T06:00:00Z
demo:FIC101.PV  37 rule hits in 300 samples

baseline  2024-03-30 20:00:00Z -> 2024-03-31 01:00:00Z   GOOD 261   censored no
window    2024-03-31 01:00:00Z -> 06:00:00Z
limits    center 62.32   sigma 0.5488   lcl 60.67   ucl 63.97
basis     individuals 3-sigma

BEYOND_3SIGMA  29
  2024-03-31 02:01:00Z -> 02:29:00Z   29 samples

RUN_9_SAMESIDE  6
  2024-03-31 01:08:00Z   9 consecutive points on one side of center 62.32
  2024-03-31 01:58:00Z   9 consecutive points on one side of center 62.32
  2024-03-31 02:09:00Z   9 consecutive points on one side of center 62.32
  2024-03-31 02:38:00Z   9 consecutive points on one side of center 62.32
  2024-03-31 04:36:00Z   9 consecutive points on one side of center 62.32
  (+1 more run)

TREND_6  2
  2024-03-31 01:25:00Z   6 consecutively increasing/decreasing points
  2024-03-31 05:56:00Z   6 consecutively increasing/decreasing points

Line by line

  • demo:FIC101.PV 37 rule hits in 300 samples: hits of all three rules over the window's GOOD samples. One sample can hit more than one rule.
  • baseline and window: the spans, as in the screen.
  • limits center 62.32 sigma 0.5488 lcl 60.67 ucl 63.97: the center line, sigma, and the lower and upper control limits at 3 sigma, in m3/h.
  • basis individuals 3-sigma: an individuals chart, one point per sample.
  • BEYOND_3SIGMA 29: samples outside the limits. A lone hit gives the time, the value and the limits it broke, such as value 100 outside [60.67, 63.97]. Consecutive hits print as one run, here 2024-03-31 02:01:00Z -> 02:29:00Z 29 samples.
  • RUN_9_SAMESIDE 6: runs of 9 samples on one side of the center, each reported at its ninth sample.
  • TREND_6 2: runs of 6 samples rising or falling, reported from the sixth sample on.

Each section lists its first five runs and counts the rest, such as (+1 more run). A rule that found nothing prints its name and 0.

What to do next

  • BEYOND_3SIGMA hits in one long run, here 29 samples from 02:01: profile that stretch and read its Range section for clipping; see clipping and censoring.
  • RUN_9_SAMESIDE hits with few BEYOND_3SIGMA: a small shift of the operating point. compare measures its size in sigmas.
  • TREND_6 hits in a row: a ramp or a drift. Check whether an operator moved the setpoint.
  • The rules, and why sigma comes from the MAD: SPC rules.