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The ‘Centre Effect’ and Statistical Process Control

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The ‘Centre Effect’ and Statistical Process Control

Alex Hodsman

Liv RI – Rank 31

Chester – Rank 35

- Identify ‘meaningful’ differences between centres
- Identify improvement/deterioration
- Multiple simultaneous comparisons
- Make fair comparisons
- (Identify modifiable clinical processes)

- Inter centre variability in outcome measures
- Chance
- Data quality
- Definitions
- Case mix
- Quality of care
- Organisational structure
- Processes of care

- Intra centre variability in outcome measures

SPC

- Method of monitoring, controlling improving a process through statistical analysis
- Key principles
- Variability in all systems
- Differentiate ‘special cause’ from ‘normal random’ variation
- Identify and improve processes to reduce special cause variation

- Cross sectional
- Funnel plots

- Longitudinal
- Control charts
- CUSUM, EWMA, SPRT etc..

- Hybrid
- Funnel plots

- Specificity
- False positive rate/Type 1 error
- 3SD = 0.27%
- 2SD = 5%

- Type 1 error
- 25 data points
- 3SD = 6.5%
- 2SD = 27.7%

- Shewhart’s original rule
- > 3SDs from the process average

- Numerous additional rules
- Patterns/Trends in the data
- E.g. 7 points in the same direction
- Enhance sensitivity
- Probability calculations

2004 Report

Funnel plot of age adjusted 1 year after 90 days survival, 2002-2005 cohort

2006 Report

Funnel plot of % with serum phosphate<1.8mmol/L:HD

Cross sectional

Inter centre variability

Good for looking at stable unit characteristics

Data, Case mix, Organisational structure

Longitudinal

Intra centre variability

Good for looking at less stable unit characteristics

Data, Processes of care

- Data collection
- Define specification of audit measure

Funnel plot to compare all centres

- Individual control chart for each centre
- Updated quarterly
- P chart - % achieving audit measure
- XMR chart for mean
- XMR chart for SD
- ? Also include a measure of process capability

Identify and analyse outliers

Check data against local audit data

Data correct

Data incorrect

- Investigate causes
- Case mix
- Quality (organisational structure)

- Investigate causes
- Quality (processes of care)

Refer to control chart to identify time of UKRR fault

- Methodical diagnostic approach to performance
- Takes chance out of the equation
- Focus resources
- Statistics are complex but the output is user friendly
- Limited ability to compare centres longitudinally i.e. rate of change