c chart for the number of nonconformities

Constructs a c chart for counts of nonconformities (defects) per inspection unit, where the unit (area, length, time, etc.) is constant across observations. For variable inspection size use shewhart_u().

Usage

shewhart_c(
  data,
  defects,
  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 the count of nonconformities per inspection unit.

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_c.

References

Montgomery, D. C. (2019). Introduction to Statistical Quality Control (8th ed.). Wiley. Chapter 7.3.

Ryan, T. P. (2011). Statistical Methods for Quality Improvement (3rd ed.). Wiley. Chapter 6 (on the inadequacy of 3-sigma limits for low-mean Poisson counts).

Examples

set.seed(1)
df <- data.frame(
  unit    = 1:40,
  defects = rpois(40, lambda = 6)
)
fit <- shewhart_c(df, defects = defects, index = unit)
#> Warning: c_bar = 6.08 is small; the normal approximation is poor.
#> ℹ Consider `limits = "poisson"` for exact limits.
fit_exact <- shewhart_c(df, defects = defects, index = unit,
                        limits = "poisson")