What’s EPB? What’s so good about RCTs?

(And what are we reading?)

Philosophy of Social SciencePhil 152 Winter 2011

Week 8

2

A theory of evidence for use

• Foundation for a guide by C&H for the use of evidence in evaluating policy effectiveness.

• The (C&H) guide should be – – Simultaneously well-grounded and practicable. – More comprehensive than those currently available – For policy-makers not expert in natural and social

science with limited amounts of time and resources.

3

The question of effectiveness

Will the proposed policy produce the targeted outcomes were it to be implemented in the targeted setting in the way it would in fact be implemented there?

So, what’s evidence – good evidence – for answers?

4

The rise of evidence-based policy

 In the UK, USA and increasingly in Europe we see a huge drive

• to use evidence to inform policy and practice • mandated by executive and legislative branches• at international, national and local levels• pushed by national and international organisations like

the Campbell and Cochrane Collaborations • with accompanying institutions and regulations to

ensure evidence is appropriately considered.

5

Some guides for use of evidence to judge policy effectiveness

• IARC: International Agency for Research on Cancer• SIGN: Scottish Intercollegiate Guidelines Network• What Works Clearinghouse• USEPA: US Environmental Protection Agency• CEPA: Canadian Environmental Protection Act• Cochrane Collaboration• Oxford Centre for Evidence-Based Medicine• Daubert decision!! (US Supreme Court)

6

For an evidence-based policy…

We want

–Evidence of high quality

–That speaks for the policy.

7

High Quality Evidence

We do not want to build an argument for a policy on shaky premises.

8

High Relevance Evidence

No matter how sturdy this foundation:

It won’t support these houses:

9

High Relevance v High Quality Evidence

(From Martin Harwit, lecture, 12 Nov 2007, IAS Durham. Ital added.)

____________________________________________

The Weight of Argument

When do symmetry arguments provide the best evidence?

When are detailed calculations found more convincing?

When does modeling appear conclusive?

For how much does societal acceptance account?

_____________________________________________

not QUALITY: The likely truth of evidence.

but RELEVANCE: Evidence that supports the conclusion.

10

High Quality Evidence We want to admit as evidence only claims that are true, or highly probable: P(e) is high. Ranking schemes rank evidence according to the method by which it is produced. Top-ranked evidence is produced by methods that make it likely that the result is true: P(e) is high.

11

Evidence-ranking Schemes: SIGNS (Scottish Intercollegiate Guidelines Network)

12

1 See Glossary 2 See Glossary 3 See Glossary

Levels of evidence

1++ High quality meta analyses1, systematic reviews2 of RCTs3, or RCTs with a very low risk of bias

1+ Well conducted meta analyses, systematic reviews of RCTs, or RCTs with a low risk of bias

1 - Meta analyses, systematic reviews of RCTs, or RCTs with a high risk of bias

2++ High quality systematic reviews of case-control or cohort studies High quality case-control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal

2+ Well conducted case control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal

2 - Case control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal

3 Non-analytic studies, e.g. case reports, case series

4 Expert opinion

13

The RCTRCTs establish causal conclusions:• In Tennessee reducing class size caused better reading scores.• In two Zambian hospitals introducing tri-co… increased survival rates

in HIV-positive children.

If properly done, they guarantee• That there was the difference indicated.• That it was caused in the way indicated (and not, e.g., by

accident or by some other factor introduced at the same time).

The method itself makes it very probable that the resulting conclusion is true.

14

What’s an RCT for ‘T causes O’?• A Mill’s method-of-difference study:

– 2 groups where all causal factors for O are distributed the same except for T and it’s downstream effects.

– T is universally present in the treatment group, universally absent in the control.

• If ProbT(O) > ProbC(O), T must be responsible for the difference.

• SO: T was a contributing cause towards O in at least some members of study population.

15

What’s special about RCTs?• They clinch causal conclusions: If the

assumptions for the study are met and ProbT(O) > ProbC(O), it follows deductively that T causes O in some study members.

• They are self-validating:– Blinding (quadruple maybe)– Random assignment– Placebo control

aim to make the distribution of other factors the same in treatment and control groups.

16

Other methods…

• Can be clinchers but none are self validating: they require substance-specific assumptions. The assumptions of the RCTs are supposedly all based on method.– Econometric modelling– Process tracing– Derivation from theory

• Some methods merely suggest a conclusion or vouch for it.– Causal relations from analogue systems– ‘Observational’ studies’.

• Standard advice guides tell you to ignore other clinchers and vouchers.

17

Daft advice

HARD WONKNOWLEDGE

18

The RCT

• The RCT can produce high quality claims – claims we have good reason to judge true – that we may adduce as evidence.

• But what are they evidence for?

• That’s our second requirement. We want high quality claims that speak for the policy. Showing that a claim is very likely true goes no way to showing that it is relevant to the truth of our policy hypotheses.

19

Turn now to Relevance

How do we decide which evidence supportswhich conclusions?

20

US Dept of Education Website

• Strong evidence for your policy = two or more high quality RCTs in ‘settings similar to that of your schools/classrooms’.

• Later elaboration adds 4 lines – trials on white suburban populations do not constitute strong evidence for large inner city schools serving primarily minority students.

21

SIGNS Grades of recommendation

A

At least one meta analysis, systematic review, or RCT rated as 1++, and directly applicable to the target population; orA systematic review of RCTs or a body of evidence consisting principally of studies rated as 1+, directly applicable to the target population, and demonstrating overall consistency of results

BA body of evidence including studies rated as 2++, directly applicable to the target population, and demonstrating overall consistency of results; orExtrapolated evidence from studies rated as 1++ or 1+

CA body of evidence including studies rated as 2+, directly applicable to the target population and demonstrating overall consistency of results; orExtrapolated evidence from studies rated as 2++

D Evidence level 3 or 4; orExtrapolated evidence from studies rated as 2+

22

Practical advice about relevance???

So—look for high quality evidence (1++) that is ‘directly applicable to the target population’!!!!

For betteradvice read Cartwright & Hardie

23

24

RCTs cannot hand evidentiary support directly to effectiveness claims