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Use Process Capability to Ensure Product Quality
Daniel Y. Peng, Ph.D.Senior Product Quality Reviewer/QbD LiaisonOffice of Pharmaceutical Sciences, CDER/FDA
IFPAC 2014 Annual MeetingArlington, Virginia January 23, 2014
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Disclaimer
This presentation reflects the views of the presenter and should not be construed to represent FDA’s views or policies.
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Concept of Process Capability First introduced in Statistical Quality Control
Handbook
by the Western Electric Company (1956). – “process capability”
is defined as “the natural or
undisturbed performance after extraneous influences are eliminated. This is determined by plotting data on a
control chart.”
ISO, AIAG, ASQ, ASTM ….. published their guideline or manual on process capability index calculation
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Four indices:– Cp
: process capability index– Cpk
: minimum process capability index
– Pp
: process performance index– Ppk
: minimum process performance index
Nomenclature
ASTM E2281: Standard Practice for Process and Measurement Capability Indices
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Calculation Formula
Cpk
= min (Cpkl, Cpku) Ppk
= min (Ppkl, Ppku)
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)( LSLUSLCp
SDLSLUSLPp 6
)(
3
LSLMeanCpkl
3
MeanUSLCpku
SDLSLMeanPpkl
3
SDMeanUSLPpku
3
USL: upper specification limit; LSL: lower specification limit;Mean: grand average of all the dataSigma hat: estimated inherent variability
(noise) of a stable processSD: overall variability
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A Perfectly Centered Process…USL
LSL
-5 -4 -3 -2 -1 0 1 2 3 4 5
LSLUSL
For this case:
Cp= 1.333Cpku=1.333Cpkl=1.333Cpk=1.333
Mean (μ ), Sigma (σ)
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Process Mean is not Centered…
When the process is not centered, or deliberately run off‐center for economic
reasons, or only a single specification limit is involved, Cpk
should be used.
Similarly, Ppk offsets Pp weakness by introducing process mean in the
calculation formula.
For this case:
Cp= 1.333Cpkl
= 1.667
Cpku
= 1.0Cpk= 1.0
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Difference between Cpk
and Ppkinherent variability overall variability
N
i
i
NXXSD
1
2
1)(
422 cSor
dMRor
dR
SD: standard deviation of all individual (observed) values, which accounts for both common cause variability (noise) and special cause
variability. It is often referred as overall variability.
: the inherent variability
(noise) due to common cause
of a stable process. It is often estimated by using within subgroup variability
which is linked to the use of control charts.
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Difference between Cpk
and Ppk
Cpk represents the potential process capability (i.e. how well a given process
could perform
when all special causes have been eliminated).
Ppk addresses how the process has performed
without the demonstration
of the process to be stable.
Cp‐Cpk or Pp‐Ppk Difference: adjust process mean
Cpk‐Ppk difference: process is not stable, identify/eliminate special causes
to reduce variability
Forecast future batch failure rate: Cpk (Yes)
; Ppk (No)– Sufficient # of samples – State of statistical control (stable process) – Normal distribution or can be transformed to normal distribution or use
reference interval (ISO 21747)
These four indices provide complimentary information.
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Control Chart
To evaluate if a process is in a state of statistical control
– Western Electric 8 Rules Two Types of Control Chart
– Variable control chart: continuous numeric measurements (e.g. assay, dissolution, uniformity, impurity level)
– Attribute control chart: discrete data (pass or fail, or counts of defects)
CL: the grand averageUCL and LCL:
• Typically: 3SD from CL•
Should not be confused
with upper and lower
specification limits
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Variable Control ChartThe average chart (X‐bar chart)The variability chart
– Moving range chart (MR chart, n=1) – Range chart (R‐chart, subgroup size 2‐10)– Standard deviation chart (S‐chart, subgroup size >10)
The average and variability charts are usually prepared and analyzed in pairs.
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Example Xbar‐R Chart
252321191715131197531
102
100
98
Batch No.
Subg
roup
Mea
n
__X=100.287
UCL=102.108
LCL=98.466
252321191715131197531
4
2
0
Batch No.
Subg
roup
Ran
ge
_R=1.78
UCL=4.582
LCL=0
252015105
104
102
100
98
96
Batch No.
Assa
y (%
)
1041021009896
LSL USL
LSL 96USL 104
Specifications
1051029996
Within
Overall
Specs
StDev 1.051Cp 1.27Cpk 1.18PPM 229.14
WithinStDev 1.079Pp 1.24Ppk 1.15Cpm *PPM 323.15
Overall
Process Capability Analysis of Tablet Assay (first 25 batches, subgroup size =3)Xbar Chart
R Chart
Run Chart
Capability Histogram
Normal Prob PlotA D: 0.636, P: 0.094
Capability Plot
Data source: Chopra, V., Bairagi, M., Trivedi, P., et al., “A case study: application of statistical process control tool for
determining process capability and sigma level,”
PDA J Pharm Sci and Tech,
66 (2), 2012, pp. 98‐115
Cp: 1.27
Cpk: 1.18
Ppk: 1.15
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Attribute Control Chart
Control chart for fraction
occurrence of an event (p chart)
– For example: % of unsuccessful batch at Site A every month– Binominal distribution
Control chart for counts
of occurrence in a defined time or
space increment (c chart)– For example: number of particulate matter in an injection vial– Poisson distribution
Other types of control chart: – cumulative sum control chart (CUSUM) – exponentially weighted moving average control charts (EWMA)– etc.
ASTM E2587‐
Standard Practice for Use of Control Charts in Statistical Process Control
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Example P chart
252321191715131197531
0.15
0.10
0.05
0.00
Month
Pro
po
rtio
n
_P=0.0437
UC L=0.1809
LC L=0
252015105
6
5
4
3
2
Month
Cu
mu
lati
ve
Un
succ
ess
Ra
te
Upper C I: 1.9123
%Defectiv e: 4.37Lower C I: 2.79Upper C I: 6.49Target: 0.00PPM Def: 43726
Lower C I: 27917Upper C I: 64891Process Z: 1.7090Lower C I: 1.5150
(95.0% confidence)
Summary Stats
302520
20
10
0
T otal Batch Manufactured/Month
% U
nsu
cce
ss R
ate
129630
10.0
7.5
5.0
2.5
0.0
% Unsuccess Rate
Fre
qu
en
cy
Tar
Binomial Process Capability Analysis of Unsuccess BatchP Chart
Tests performed w ith unequal sample sizes
Cumulative Unsuccess Rate
Unsuccess Rate
Histogram
Similar principles can be used to evaluate process capability of
a single product, a product
class, different manufacture sites, or a manufacturer global sites.
Process‐Z: 1.709
Binomial process
capability index:
0.569
% of “unsuccessful batch”/month at Site A (# of lots attempted: 20‐30/month)
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Sample Size and Sampling Frequency
Sample size:– Law of large numbers: >30 – ISO 8528: at least 25 subgroups– ASTM E2587:
• At least 100 numeric data points ( subgroup size > 1) (for example 6
tablets/batch dissolution test, total batch> 17)
• At least 30 numeric data points (subgroup size = 1)• Attribute data: 20 to 25 subgroups of data are suggested
– Uncertainty of the index : e.g. lower 95% confidence bound
Sampling frequency:– Not to violate the randomness assumption – Minimize the variation of observations within a subgroup and to
maximize variation between subgroups
– Average run length (ARL) of the control charts
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Potential Applications 1. Product/Process Design and Understanding Stage
– Deliberately change input material attributes and process parameters according to experimental design
– Identify special causes and develop control strategy to eliminate or reduce variability
Question for Discussion:– Does your organization have any program to calculate
preliminary Ppk? (Yes/No and Comments/Suggestions)?– What would be the criterion to show that the designed
formulation /process is ready for scale up and tech transfer?
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Potential Applications
2. Process Scale up and Qualification Stage:– To establish scientific evidence that the process is reproducible at
commercial scale and the process will consistently deliver a product
that meets the quality standard established in development stage
Challenges: limited commercial scale batches
Higher level of sampling and testing to demonstrate
product quality
Once sufficient data points are collected – Evaluate if the process reaches a stable state– Establish control limits for control chart– Calculate Cpk and its 95% lower confidence bound
Question for discussion:– What would be a reasonable acceptance criterion to define an
acceptable process? For example Cpk 95% lower confidence bound >1
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Potential Applications 3. Routine Commercial Manufacturing Stage
– continual assurance that the process remains in a state of control (the
validated state) during commercial manufacture
Traditional sampling and testing to confirm
product quality
Statistical process control tools to monitor the process
Cpk/Ppk monitoring and trending
Question for Discussion:– Cumulative or last 25 or 30 batches? or time based (monthly,
quarterly, annually)?
– Continual improvement opportunity? (freedom vs. responsibility)– When and How often to re‐evaluate the control limits?
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Looking at Product Quality: Case Study
Case study: an antiepilepsy
drug product
Dosage form: Immediate Release Tablet, 100 mg strength
Data: annual stability batch release data from the Reference
Listed Drug (RLD) and 8 generic products are collected. – # of batches: 10‐18– All examined brand and generic batches met current USP
quality standards.
CQA selection for process capability analysis– Excellent chemical stability was demonstrated for all the
products (<ICH Reporting Threshold)
– Content uniformity data is not available in the annual report– Performed process capability analysis for Assay
and Dissolution
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Looking at Product Quality: Case StudyProducts # of
Batches
Assay (subgroup size=1) Dissolution
Ppk* Ppk‐CL* Ppk Ppk‐CL Subgroupsize
USP Test
RLD 18 3.50 2.51 3.84 3.41 6 Test 3Generic‐1 11 2.47 1.55 3.07 1.93 1 Test 3Generic‐2 10 1.98 1.19 1.37 0.81 1 Test 2Generic‐3 11 2.84 1.78 2.78 1.74 1 Test 1Generic‐4 10 4.51 2.76 3.80 2.32 1 Test 2Generic‐5 11 3.15 1.98 1.36 1.15 6 Test 1Generic‐6 10 5.54 3.38 9.94 6.08 1 Test 3Generic‐7 13 2.63 1.73 2.66 2.30 6 Test 1Generic‐8 11 1.22 0.74 3.31 2.08 1 Test 3
* Sample size is limited and none of products demonstrated statistical control state for
both CQAs. Hence, Ppk and its lower 95% confidence bound (Ppk‐CL) are reported.
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Summary: Process Capability Indices
Patient first: clinical relevant specification
Consider not only process mean & variability but also in relation
to the specification
Quantitative and action enabling
Applicable for cross sectors (brand, generic, OTC and biotech)
No additional testing is required since batch release data is
available per current regulation
A simple and powerful indicator to ensure product quality and
process robustness.