Hss4303b – intro to epidemiology
April 8, 2010 – epidemiology and health policy
Exam Review
• April 22, 2pm-3:30pm, SCS E218
• April 26, 2pm-3:30pm, SCS E218
Poster Sh*t
• For those printing
their posters with
the Geography dept
or Merriam Print,
the deadline is
TODAY in order to
assure pick-up by
Saturday
Important Poster Sh*t
• The agenda and list of presenters is now
posted on the website
• Your presentation time is also listed
• If you are scheduled to present in the
afternoon, you are still encouraged (dare I say,
required?) to register in the morning
– If you are not there to present to the judges when
they come around, you will receive zero
What is this?
OTTAWA - Health Canada is advising Canadians about important safety
information for CRESTOR® (rosuvastatin). A recent US study has found that
Asian patients may be at greater risk of developing muscle-related adverse
events with this drug. CRESTOR® is a cholesterol-lowering drug in the
"statin" family. "Statins" are a specific type of cholesterol-lowering medication.
In Canada, and internationally, CRESTOR® has been associated with reports
of a serious condition called rhabdomyolysis. Rhabdomyolysis results in
muscle breakdown and the release of muscle cell contents into the
bloodstream.
Symptoms of rhabdomyolysis include muscle pain, weakness, tenderness,
fever, dark urine, nausea, and vomiting. In severe cases, rhabdomyolysis can
lead to kidney failure and be life-threatening.
For some patients, there may be pre-existing conditions or other personal
factors that could cause a greater risk of developing muscle-related problems,
including rhabdomyolysis, if they are using "statin" medications.
RISK
The techniques of epidemiology are
used to collect data and create
information to quantify risk in order
to allow more informed policy.
What is health policy?
Dark blue slides are from Dr Spasoff, supercourse
Light blue slides by Dr Akram, supercourse
Policy is like sausage: it may taste
good, but it’s best that you don’t know
what went into it
Epidemiology contributes at each step
“What if” questions
• “What if” questions like “What would be the
effect on the overall health of the population
if we reduced smoking by 20%?
• Sort of like program evaluation
Clinical Decision Making
• In a clinical medical environment, sometimes
we need to use evidence to quantify our
decision-making process
• Eg, to choose one therapy over another
Decision Tree
Also called “chance node”
Also called “choice node”
Data for Decision Tree
• Epidemiology
– Probabilities of outcomes
– Meta-analyses
– Systematic reviews
– Analytical studies
– Pilot studies
Example Slides by Dr James Kahn, UCSF, 2010 “Decision Analysis”
Motivating Case:
Ms. Brooks is a 50 year old woman with an
incidental cerebral aneurysm. She presented with
new vertigo 3 weeks ago and her primary MD
ordered a head MRI. Her vertigo has subsequently
resolved and has been attributed to labyrinthitis.
Her MRI suggested a left posterior
communicating artery aneurysm, and a catheter
angiogram confirmed a 6 mm berry aneurysm.
Case Presentation (cont’d)
Past medical history is remarkable only for 35 packyears of cigarette smoking.
Exam is normal.
Ms. Brooks: “I don’t want to die before my time.”
Question is: Do we recommend surgical clipping of
the aneurysm or no treatment?
Overview of DA Steps
1. Formulate an explicit question
2. Make a decision tree.
(squares = decision nodes, circles = chance nodes)
a) Alternative actions = branches of the decision node.
b) Possible outcomes of each = branches of chance nodes.
3. Estimate probabilities of outcomes at each chance node.
4. Estimate utilities = numerical preference for outcomes.
5. Compute the expected utility of each possible action
6. Perform sensitivity analysis
1. FORMULATE AN EXPLICIT QUESTION
- Formulate explicit, answerable question.
- May require modification as analysis progresses.
- The simpler the question, without losing important
detail, the easier and better the decision analysis.
In the aneurysm example, our interest is in determining
what’s best for Ms. Brooks so we'll take her perspective. We
will begin with the following question:
Which treatment strategy, surgical clipping or no
treatment, is better for Ms. Brooks considering her primary
concern about living a normal life span?
2. MAKE A DECISION TREE
Creating a decision tree = structuring the problem
Provide a reasonably complete depiction of the
problem.
Best is one decision node (on the left, at the
beginning of the tree).
Branches of each chance node -- exhaustive and
mutually exclusive.
Proceed incrementally. Begin simple.
Simple Tree
Normal survival
No treatment
Early Death
Ms. Brooks
Surgery:
yes or no?
Normal survival
Surgery
Early Death
More complicated tree
Surgery:
yes or no?
No
Surgery
Normal survival
Yes
Early Death
No
Normal survival
Yes
Early Death
Aneurysm
Rupture?
No treatment
Ms. Brooks
No
Aneurysm
Rupture?
Surgical
Death?
Yes
Early Death
Crazy complicated
Normal survival
No
Aneurysm
Rupture?
No treatment
Yes
Ms. Brooks
No
Normal survival
Yes
Early Death
Death?
Surgery:
yes or no?
Normal survival
No
No
Aneurysm
Rupture?
Yes
Surgery
Surgical
Death?
Yes
No
Normal survival
Yes
Early Death
Death?
Early Death
3. Fill in the Probabilities
• Use info from the literature
– Case fatality rates
– Population mortality rates
– etc
No
p=0.9825
No treatment
Aneurysm
Rupture?
Yes
p=0.0175
Ms. Brooks
Normal survival=1
No
p=.55
Normal survival=1
Yes
p=.45
Early Death=0
Death?
Surgery:
yes or no?
No
p=1.0
No
p=0.977
Aneurysm
Rupture?
Yes
p=0
Surgery
Surgical
Death?
Yes
p=0.023
Normal survival
No
p=.55
Normal survival
Yes
p=.45
Early Death
Death?
Early Death
Expected Utility
• The average or expected outcome if one
follows a given branch of the tree
• Sum of desirable outcomes within a given
branch
Example of Expected utility
• Disease = cardiac valve failure
• Intervention (decision) = surgery vs no surgery
• If surgery, possible outcomes are:
complications vs no complications
– Further possible outcomes are death or survival
• If no surgery, the only possible outcomes are
death or survival
Example of Expected utility
• Let’s follow surgery node:
– 90% chance of no complications
• 90% survive
– 10% chance of complications
• 50% survive
• What is expected utility at the surgery node?
Example of Expected utility
• Let’s follow surgery node:
– 90% chance of no complications
• 90% survive
– 10% chance of complications
• 50% survive
EU = (P of no complications)(survival) + (P of complications)(survival)
= 0.90 x 0.90 + 0.10 x 0.50
= 0.81 + 0.05
= 0.86
COMPUTE THE EXPECTED UTILITY OF EACH
BRANCH
Called "folding back" the tree.
Expected utility of action = each possible
outcome weighted by its probability.
Simple arithmetic calculations
Back to Ms Brooks
(Using a fairly complex system that I won’t expect you to duplicate)
5. Compute expected utility of each branch
No
p=0.9825
No treatment
Aneurysm
Rupture?
Yes
p=0.0175
Ms. Brooks
Normal survival=1
No
p=.55
Normal survival=1
Yes
p=.45
Early Death=0
=.55
Death?
=0
Surgery:
yes or no?
No
p=1.0
No
p=0.977
Aneurysm
Rupture?
Yes
p=0
Surgery
Surgical
Death?
Yes
p=0.023
Normal survival=1
No
p=.55
Normal survival=1
Yes
p=.45
Early Death=0
=.55
Death?
Early Death=0
=0
5. Compute expected utility of each branch
No
p=0.9825
No treatment
Aneurysm
Rupture?
=.9921
Yes
p=0.0175
Ms. Brooks
Normal survival=1
=.9825
No
p=.55
Normal survival=1
Yes
p=.45
Early Death=0
Death?
=.55
Surgery:
yes or no?
=1.0
No
p=1.0
No
p=0.977
Aneurysm
Rupture?
No
p=.55
Yes
p=0
Surgery
Normal survival=1
Surgical
=.977
Death?
Death?
=.55
Yes
p=.45
Yes
p=0.023
Normal survival=1
Early Death=0
Early Death=0
=0
Ms. Brooks
We recommend NO surgery.
• “Thanks… But I meant I wanted to live the
most years possible. Dying at age 80 isn’t
as bad as dying tomorrow…”
Improve the Analysis
Add layers of complexity to produce a more
realistic analysis.
Eg: Add Another Outcome
Three outcomes
Determine utility as a portion
of expected life span
No treatment
Aneurysm
Rupture?
-Normal survival 1.0
-Early death 0.5
-Immediate death 0
Ms. Brooks
Normal survival
No
Yes
No
Normal survival
Yes
Early Death
Death?
Surgery:
yes or no?
Normal survival
No
No
Aneurysm
Rupture?
Yes
Surgery
Surgical
Death?
Yes
No
Normal survival
Yes
Early Death
Death?
Immediate Death
Summary of Formal Decision
Analysis
•
•
•
•
•
Explicit question.
Decision tree.
Probabilities of each outcome.
Utilities for each outcome.
Expected utility of each course of
action.
• Sensitivity analysis.