u chart for nonconformities per unit, variable inspection size

Constructs a u chart from defect counts and a per-observation "exposure" (inspection size: square metres of fabric, hours of operation, lines of code, etc.). For constant exposure use shewhart_c().

Usage

shewhart_u(
  data,
  defects,
  exposure,
  index = NULL,
  limits = c("3sigma", "poisson"),
  rules = c("nelson_1_beyond_3s"),
  locale = getOption("shewhart.locale", "en"),
  verbose = NULL
)

Arguments

data

A data frame.

defects

Tidy-eval column reference for raw defect counts.

exposure

Tidy-eval column reference for the inspection size (must be positive).

index

Optional tidy-eval column reference for the x-axis.

limits

One of "3sigma" (default; classical normal approximation) or "poisson" (exact Poisson 0.00135 / 0.99865 quantiles, recommended when c_bar < 10).

rules

Character vector of rule keys to apply. See shewhart_rules_available(). Default applies Nelson 1 and 2.

locale

One of "en", "pt", "es", "fr". Affects plot labels and informative messages.

verbose

Logical. Print progress messages? Defaults to the shewhart.verbose option.

Value

A shewhart_chart object of subclass shewhart_u.

References

Montgomery, D. C. (2019). Introduction to Statistical Quality Control (8th ed.). Wiley. Chapter 7.3.2.

Examples

set.seed(1)
df <- data.frame(
  roll     = 1:25,
  defects  = rpois(25, lambda = 4 * runif(25, 0.5, 1.5)),
  m2       = runif(25, 0.5, 1.5)    # variable inspection size
)
fit <- shewhart_u(df, defects = defects, exposure = m2, index = roll)