![Page 1: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/1.jpg)
Interpreting numbers – more tricky bitsScotPHO training course
March 2011
Dr Gerry McCartney
Head of Public Health Observatory Division
NHS Health Scotland
![Page 2: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/2.jpg)
Content
• More on causality• Attributable fractions• Screening – pitfalls to watch out for
![Page 3: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/3.jpg)
Does A cause B?
A B
A B
A
B
A B
C
?
![Page 4: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/4.jpg)
Factors which make causality more likely
Bradford-Hill criteria• Strength of association• Consistency• Specificity• Temporality• Biological gradient• Plausibility• Coherence• Experiment• Analogy
![Page 5: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/5.jpg)
CoffeeIschaemic
heart disease
Does coffee cause ischaemic heart disease?
![Page 6: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/6.jpg)
• Factors that do not lie on the causal pathway but which influence the magnitude of effect
Effect modifiers
SmokingIschaemic
heart disease
Male gender(effect modifier)
![Page 7: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/7.jpg)
Asbestos exposure Asbestosis
Necessary or sufficient causes?
Smoking Lung cancer
Jumping from plane without parachute
Squished onto ground
![Page 8: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/8.jpg)
Attributable fractions/risk
• “What fraction of disease incidence in the exposed group is attributable to the risk factor?”
• Calculated by taking the relative risk in an unexposed group from the relative risk in an exposed group
![Page 9: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/9.jpg)
Attributable fractions
Lung cancer deaths per 1,000 population per year
Coronary heart disease deaths per 1,000 per year
Heavy smokers 166 599
Non-smokers 7 422
![Page 10: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/10.jpg)
Attributable fractions
Lung cancer deaths per 1,000 population per year
Coronary heart disease deaths per 1,000 per year
Heavy smokers 166 599
Non-smokers 7 422
Excess risk of heavy smoking
166 – 7 = 159 599 – 422 = 177
![Page 11: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/11.jpg)
Attributable fractions
Lung cancer deaths per 1,000 population per year
Coronary heart disease deaths per 1,000 per year
Heavy smokers 166 599
Non-smokers 7 422
Excess risk of heavy smoking
166 – 7 = 159 599 – 422 = 177
Attributable risk of heavy smoking
159 / 166 = 95.8% 177 / 599 = 29.5%
![Page 12: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/12.jpg)
Attributable fractions/risk
Attributable fractions can also be applied to the whole population using the formula:
= (risk in total population – risk in unexposed population) / risk in total population
![Page 13: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/13.jpg)
Screening
• Why do we screen for conditions? • When is screening appropriate?• Problems with evaluation of screening
programmes• Particular biases
![Page 14: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/14.jpg)
Why screen for conditions?
• To improve outcomes for individuals – Keep Well health checks– Breast mammography
• To improve outcomes for populations – Port health checks– Employment checks
![Page 15: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/15.jpg)
When should you screen?
Based on the Wilson – Junger criteria:
• Is there an effective intervention?• Does earlier intervention improve outcomes?• Is there a screening test which recognises disease
earlier than usual?• Is the test available and acceptable to the target
population? • Is the disease a priority? • Do the benefits outweigh the costs?
![Page 16: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/16.jpg)
Screening – why is it different?
• Individuals may not benefit• Involves people who are well subjecting themselves to testing
– medicalisation • Creation of a pre-disease state• False positive tests• False negative tests • Initiated by health professionals not individuals• Cost-benefit depends on prevalence within a population • Inequalities implications
![Page 17: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/17.jpg)
Particular biases• Lead time bias
Given that screening picks up disease at an earlier stage – the time between diagnosis and death increases without any actual increase in survival
Symptoms
Detected by screening
Death
Death
![Page 18: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/18.jpg)
• Length time bias
Screening is more likely to detect less aggressive disease and therefore can give impression of improved survival
X
X
X
X
X
X
X
![Page 19: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/19.jpg)
Measures used in screening
• Sensitivity is the likelihood that those with disease will be picked up by the screening test
• Specificity is the likelihood that those with a negative screening test will not have the disease
• Positive predictive value is the likelihood that those with a positive test will have the disease
• Negative predictive value is the likelihood that those with a negative test will not have the disease
![Page 20: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/20.jpg)
Measures for screening
• Sensitivity and Specificity
• Positive predictive value and Negative predictive value
Disease Total
Yes No
Screening test
Positive 300 30 130
Negative 20 3000 3020
Total 320 3030 3350
![Page 21: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/21.jpg)
Measures for screening
• Sensitivity and Specificity
• Positive predictive value and Negative predictive value
Disease Total
Yes No
Screening test
Positive 300 30 130
Negative 20 3000 3020
Total 320 3030 3350
Sensitivity = 300/320 = 94%
Specificity = 3000/3030 = 99%
PPV = 300/330 = 91%
NPV = 3000/3020 = 99%
![Page 22: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/22.jpg)
![Page 23: Interpreting numbers – more tricky bits ScotPHO training course March 2011 Dr Gerry McCartney Head of Public Health Observatory Division NHS Health Scotland](https://reader035.vdocuments.mx/reader035/viewer/2022062718/56649e735503460f94b7329e/html5/thumbnails/23.jpg)
Summary
• Bradford-Hill criteria can be used to judge whether an association is likely to be causal
• Attributable fractions can help identify the discrete contribution of particular risks to an outcome
• Screening is different to other medical interventions and can cause harm
• Screening evaluations have their own potential biases – lead time and length time bias