Decision Analysis

Dr M G DawesCentre for Evidence Based

Medicine

Contents What is Decision Analysis? Decision Trees How they work Critical Appraisal of Decision

Analysis A bit on economics An example of some software

What is Decision Analysis? Urinary Tract Infection Patient presents with symptoms What would happen if you opted

for one path in preference to another

On what basis would you assess outcome

How does one start? List all the options and display: Commonly accepted format is a

tree diagram.UTI

Decision Node

Treat

Don’t Treat

Decision data

UTI

Chance Node

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

Result 0.9 is better than 0.2

UTI

Chance Node

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

Add reality to probability scores Cost of tests Cost of treatment Days lost from work Cost of re attending clinician

Multiply the probabilities by the costs

Cost Data

UTI

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

2

9

2

9

Costs

Rollback Costs

UTI

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

2

9

2

9

(2*0.9) + (9*0.1)=2.7

(0.2*2)+(0.8*9)=7.6

Results More people get better (90 % vs

20%) It is cheaper (2.70 vs 7.60)

What does the patient think? Utilities How would you feel? QALYs Quality adjusted life year

QALY’s

Survival in years

01 2 3 4 5 6

1.0

Qualityof life

valuation

Health profile with intervention

Health profilewithout intervention

Quality adjusted life years gained

From Alastair Gray

Time Trade Off Method (to assess utility of a health state) You have arthritis (severe - unable to

walk to shops – need a buggy – in pain most of the time) and are aged 48

Choose between living with arthritis until 80 or living in perfect health for a shorter length of time

eg 50 60 70 If 70 – 65 67 69 71 73 75 Etc until chosen a year

Time Trade Off Utility is 1- (number of years

willing to give up/(80-current age) If age selected was 75 Utility = 1-(80-75)/(80-48) = 0.84 The better your health the less the

years you give up

Trade Off vs AgeTrade off calculation of utility

00.10.20.30.40.50.60.70.80.9

1

40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72

Age (years)

Uti

lity

Trade off 2 Trade off 4 Trade Off 6

Utilities

UTI

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

9

2

9

2

Utilities

Rollback

UTI

Treat

Don’t Treat

better

Not better

better

Not better

0.9

0.1

0.2

0.8

9

2

9

2

(9*0.9) + (2*0.1)=8.3

(0.2*9)+(0.8*2)=3.4

Results More people get better (90 % vs

20%) It is cheaper (2.70 vs 7.60) The utilities are better (8.3 vs 3.4) Probably should treat?? Sensitivity analysis

UTI: What are the options? Treat on symptoms alone Treat after doing a test Exercise : what are the options Diagnosis Therapy Outcome

How does one develop this? Need to know all the baseline data. For UTI What proportion of patients with

typical symptoms have UTI What is the sensitivity and

specificity of a dipstix. What is the success rate of

antibiotic treatment?

On what basis would you assess outcome? Bacterial eradication Symptoms

Diary Questionnaire Interview

Time off work What do patients think?

How are these studies appraised Are the results valid What are the results Can these be applied to my

patients

Are the results valid? Were all important strategies and

outcomes included? Were all of the realistic clinical strategies

compared? Were all clinically relevant outcomes

considered? Was an explicit and sensible process

used to identify, select and combine the evidence into probabilities?

Validity Check (2) Were the utilities obtained in an

explicit and sensible way from credible sources?

Was the potential impact of any uncertainty in the evidence determined? 

What are the Results?

In the baseline analysis, does one strategy result in a clinically important gain for patients? If not, is the result a toss-up?

 How strong is the evidence used in the analysis?

 Could the uncertainty in the evidence change the result?

Can I apply the results to my patient? Do the probability estimates fit my

patients' clinical features? Do the utilities reflect how my

patients would value the outcomes of the decision?

Bottom Line The power of decision analysis is not in

the numbers at the decision node It is the ability to change the utilities

and probabilities Watching how this affects the decision

node Thus it should be seen as a dynamic

tool Software for this is available

EXCEL or WWW.TREEAGE.COM

‘Nice’ system ‘Quite’ friendly Free download to try US$ 295 academic price