Control Charts

Robin Henderson Royal Infirmary of EdinburghMargrethe van Dijke Western General Hospital, Edinburgh

National Stroke Audit Coordinators

Charts have been used in healthcare for a very long time! Carl Wunderlich 1861

Control Charts

                                              

Florence Nightingale – the “passionate statistician” - is believed to have invented a type of chart

A patient’s temperature chart from 1963 – a run chart.

The horizontal line at 98.2 °F indicates “normal” temperature

The red horizontal lines indicate the normal range 97.5 to 98.8 °F

The run chart has now been converted to a control chart – Walter Shewhart 1924

Henderson, D. A. et al. JAMA 1999;281:2127-2137.

Typical Temperature Chart of Patient With Smallpox Infection

The control chart helps us to distinguish between “common cause” variation and “special cause” variation

Month

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Sep-06J ul-06May-06Mar-06J an-06Nov-05Sep-05Jul-05May-05Mar-05Jan-05

100

90

80

70

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50

40

30

Swallow Screening at RIE

A run chart of the monthly proportion of patients with recorded swallow screens at RIE

To turn this run chart into a control chart we have to calculate a centre line and limits. The calculations are quite easy to do but most users get a computer to perform the calculations and plot the chart. An Excel tool may be downloaded from: -

http://www.indicators.scot.nhs.uk/SPC/SPC.html

Month

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_Average = 66

Upper Limit =100

Lower Limit = 30

Swallow screening at RIE

All points fall between the tram lines and there are no “unusual” patterns of variation so there is no evidence of any special cause variation

Month

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Apr-07Jan-07Oct-06Jul-06Apr-06Jan-06Oct-05Jul-05Apr-05Jan-05

100

90

80

70

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50

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_Average = 66

Upper limit = 100

Lower limit = 30

2

Swallow screening at RIE

We have evidence of improvement! If the average had remained at 66 then nine in a row above 66 would be most unlikely

It appears that a campaign by SALT staff has been effective and increased the average monthly proportion of patients screened from 66% to 82%.

Month

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Apr-07Jan-07Oct-06Jul-06Apr-06Jan-06Oct-05Jul-05Apr-05Jan-05

100

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80

70

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50

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_Average = 82

UB=100

Lower limit = 51

1 2Swallow screening at RIE

Case Study – NV Clinic at WGH

Days from referral to examination for all patients

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Months

Days WGH Mean

WGH Median

Days from receipt of referral to examination <= 14 days for DEFCVD patients and <= 7 days,

80% Meets NHS QIS Standard for both

0102030405060708090

100

Months

Perc

enta

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<= 7 days<= 14 daysTarget

181614121086420

Date

Days

_X=3.55

UCL=6.10

LCL=1.00

1 21 21 2Mean days from referral to examination for DEFCVD patients

X=11.31_

The mean waiting time from referrals being received to patients being seen in the NV Clinic has decreased from 11.3 to 3.5.

100

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0

Date

Perc

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_X=91.5UB=100

LCL=72.9

1 21 21 21 21 21 2I Chart of Percentage of NV Clinic patients seen within 7 days

X=28.8_

The mean percentage of DEFCVD patients seen in the NV Clinic within seven days since referral has being received has increased from 28.8 to 91.5.

How do you know that your stroke care is improving?

Control charts are a very useful tool for addressing this question.

“Display is an obligation!”

John Tukey 1986

Have a go!

Thank You Very Much!

Telephone 0131 242 6934 e-mail Robin.Henderson@luht.nhs.uk