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Apply runs tests to a chart object or to raw vectors

Source: R/runs-tests.R
shewhart_runs.Rd

Implements the eight rules of Nelson (1984, 1985) plus a Western Electric "7 in a row" variant for backward compatibility. Returns a tidy tibble of rule violations.

Usage

shewhart_runs(
  x,
  rules = c("nelson_1_beyond_3s", "nelson_2_nine_same"),
  center = NULL,
  sigma = NULL
)

Arguments

x

Either a shewhart_chart object (most common) or a numeric vector of values. If a vector is supplied, center and sigma must also be provided.

rules

Character vector of rule keys to apply. Use shewhart_rules_available() to see all options. Default applies Nelson 1 (beyond 3 sigma) and Nelson 2 (9 same side), the most commonly recommended pair.

center, sigma

Numeric scalars or vectors. Required only when x is a numeric vector. Ignored otherwise.

Value

A tibble with columns position (integer, the index where the rule fired), rule (character key), description (character label), value (the value at that position) and severity (currently always "out_of_control"; reserved for future warning-level rules).

References

Nelson, L. S. (1984). The Shewhart Control Chart – Tests for Special Causes. Journal of Quality Technology, 16(4), 237-239. doi:10.1080/00224065.1984.11978921

Nelson, L. S. (1985). Interpreting Shewhart Xbar Control Charts. Journal of Quality Technology, 17(2), 114-117. doi:10.1080/00224065.1985.11978941

Western Electric Co. (1956). Statistical Quality Control Handbook.

Examples

set.seed(1)
x <- c(rnorm(20), 5, rnorm(20))    # one outlier at position 21
shewhart_runs(x, center = 0, sigma = 1)
#> # A tibble: 1 × 5
#>   position rule               description            value severity      
#>      <int> <chr>              <chr>                  <dbl> <chr>         
#> 1       21 nelson_1_beyond_3s 1 point beyond 3 sigma     5 out_of_control