Quality control chart subgroup size

Calculate number to sample for quality control chart datapoint

EXCEL functions

Data Entry

Indicator rate, as decimal
(eg 9% => 0.09; 0.77% => 0.0077; 3.27‰ => 0.00327):

Smallest denominator (during previous 12):

Collection cycle:
month quarter

Control chart type:
p, np u, c x̄

Improvement direction:
Higher Lower

Subgroup Size Selection Rules

p chart:

[Rule 1] n ≥ 5 ÷ min(p, 1- p)
[Rule 2] n ≥ ln(0.05) ÷ ln[max(p, 1-p]
[Rule 3] n > k² × max[(1-p)÷p, p÷(1-p)]
[Rule 4] n ≥ (1.4 ÷ p)

u chart:

[Rule 1] n ≥ 5 ÷ λ
[Rule 2] n ≥ -ln(0.05) / λ
[Rule 3] n > k² ÷ λ
[Rule 4] n ≥ [1.4 ÷ λ ]

Result

Rate:
Minimum subgroup size:
[Rule 1]: ~ (General rule) [0~0 ]
[Rule 2]: ~ (General rule)
[Rule 3]: [0 ]
[Rule 4]:

[Rule 3] ensure lower control limit greater than zero (LCL>0)
[Rule 4] ensure that less than 25% of data points are zero

References

Benneyan JC. Design, use, and performance of statistical control charts for clinical process improvement.
www.npaihb.org/ 2001 (pdf: 27pp)