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Control chart (SPC) templates

I-MR Chart — API p95 Latency

Thirty fifteen-minute windows of p95 latency as individuals and moving range against a 250 ms SLO, where the deploy spike inflates the moving range and drives Cpk 2.23 apart from Ppk 1.46 — the gap between short-term and long-term spread.

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p95 latency — checkout APII-MR chart · 30 points · ms · all 8 Nelson rulesI chart — individual readings1501752002252501333USL 250.0UCL 211.2CL 179.7LCL 148.2MR chart — moving range of 2020406011UCL 38.75CL 11.86LCL 0.00123456789101112131415161718192021222324252627282930Out of control — 6 of 30 points flagged by rules 1, 3.Cpk 2.23 · within-subgroup sigma 10.5 ms · 1.2e-5 PPM expected out of specPpk 1.46 · overall sigma 16.0 ms · 5.9 PPM observed-spread estimateCapable — Cpk is 1.33 or better. Quoted from an unstable process, so treat it as provisional.Rule 1 · Individual value at point 22 = 251.0 — 1 point beyond a 3σ limitRule 3 · Individual value at point 27 = 175.0 — 6 points in a row steadily increasing or decreasingRule 3 · Individual value at point 28 = 173.0 — 6 points in a row steadily increasing or decreasingRule 3 · Individual value at point 29 = 165.0 — 6 points in a row steadily increasing or decreasingRule 1 · Moving range at point 22 = 67.00 — 1 point beyond a 3σ limitRule 1 · Moving range at point 23 = 59.00 — 1 point beyond a 3σ limit

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title "p95 latency — checkout API"
chart i-mr
unit ms
# One-sided specification: the SLO is an upper bound, so only Cpu
# exists and Cpk collapses to it.
spec usl: 250
# One reading per 15-minute window; individuals because there is no
# rational subgroup in a time series of aggregates.
values 170 168 161 195 181 183 181 193 157 182
values 178 178 176 171 172 182 178 180 172 172
# 14:15 deploy — connection-pool exhaustion for one window.
values 184 251 192 187 184 176 175 173 165 174