Models to Inform Recommendations about Preventive Services (Text Version)

Slide Presentation from the AHRQ 2008 Annual Conference

On September 8, 2008, this presentation was made at the 2008 Annual Conference. Select to access the PowerPoint® presentation (146 KB).

Slide 1

Models to Inform Recommendations about Preventive Services Perspective of the United States Preventive Services Task Force.

Slide 2

An RCT for Every Topic is an Unattainable Goal: Technical

• Behavioral interventions.
  • Inability to maintain fidelity with treatment.
  • Inability to control provider effects.
• Quality improvement interventions.
  • Inconsistency of implementation.
  • Difficulty in withholding interventions with what is viewed as high face validity.
• Contamination.

Slide 3

An RCT for Every Topic is an Unattainable Goal: Technical

• Community programs.
  • Insufficient number of communities willing to be randomized.
  • Inability to maintain fidelity with program parameters.
  • Need for adaptability to gain cooperation.
  • Need for adaptability to achieve effectiveness.
  • Contamination.

Slide 4

The Parachute Problem

• Do we really NEED an RCT? "....the effectiveness of parachutes has not been subjected to rigorous evaluation by using randomised controlled trials....we think that everyone might benefit if the most radical protagonists of evidence-based medicine organized and participated in a double-blind, randomized, placebo- controlled crossover trial of the parachute...."
• Sackett. Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomised controlled trials. BMJ 2003;327:1459-61.

Notes:

• In the upper right hand corner is an image of a man parachuting.

Slide 5

But Even More Importantly

• It is not feasible to conduct an RCT for every topic for which clinical decisions must be made.

Slide 6

CRC: Options in 2008

• Something found in stool.
  • Occult blood using guaiac (gFOBT).
    • Un-rehydrated.
    • Rehydrated.
  • Occult blood using test that is more sensitive than guaiac.
  • Fecal DNA.
• Something "seen" in the colon.
  • Visual.
    • Flexible sigmoidoscopy.
      • Alone.
      • With FOBT (several options).
    • Colonoscopy.
  • Radiographic.
    • Dual contrast barium enema (DCBE).
    • CT colonography (CTC).

Slide 7

Number of RCTs: CRC Screening Type of Test

• 9 strategies against no screening.
  Would require 8 two-arm trials or a very large 8-arm trial (unrehydrated gFOBT already done).
• 9 strategies against all other possible strategies alone.
  E.g. more sensitive FOBT versus unrehydrated FOBT.
  • FOBT versus optical colonoscopy.
  • Optical colonoscopy versus CT colonography.
  • Flexible sigmoidoscopy versus fecal DNA etc. etc. etc.
  Would require 35 two-arm trials or a very large 8-arm trial.

Slide 8

It May be Impossible or Nearly Impossible To Conduct a Valid RCT

• Starting age.
  • Age 40 or 50 or something else for CRC.
  • Age 16 or 21 or something else for cervical cancer.
• Stopping age.
  • Stop age 75 versus 85 for CRC.
  • Stop age 65 versus 75 for cervical cancer.
• Screening interval.
  • Every 5 years versus every 10 years versus once in a lifetime for CRC.
  • Every year versus every 3 years versus every 5 years for cervical cancer.

Slide 9

Models: What If...

• Long-Term Financial Planning.
  • What if I live to age 95, how much money will I need to live comfortably considering inflation and the rate of return on my investments? (Make a model).
  • What do you recommend to assure that I don't run out of money? (Use the model to inform the recommendation).

Slide 10

Models: What If...

• Short-Term Financial Decisions.
  • What if I buy a car that gets 65 miles per gallon, how much money will I save over five years considering the number of miles I drive? (Make a model).
  • Should I buy a car that costs $34,000? (Use the model to inform the decision).

Slide 11

Models: What If...

• Policies.
  • What if people in developing economies use the same number of barrels of crude oil per capita as people in the United States, in what year will the world supply of crude oil be exhausted? (Make a model).
  • What does the committee recommend to assure that there is sufficient availability of energy for future generations? (Use the model to inform the recommendation).

Slide 12

Models: What If...

• Daily Decisions.
  • What if I leave the office at 3 o'clock instead of 5 o'clock, how long will it take to get back home over the Bay Bridge considering the month, the day of the week and the weather? (Make a model).
  • Should I leave early today to be sure that I am not late to dinner at the Smiths? (Use the model to inform the decision).

Slide 13

Models in the Context of Evidence-Based Medicine

• Conscientious: careful, thorough, meticulous.
  • Good models meet this criterion.
• Explicit: open, clear, plain.
  • Modelers need to do this better.
• Judicious: sensible, well thought out.
  • Good models meet this criterion.

Slide 14

Results From Good Models: Does This Information Help Make a Recommendation? The USPSTF Thinks the Answer is Yes.

The table shows the results for testing for Cancer A.

• Test for cancer A.
  • Start age: 20
  • Stop age: 65
  • Interval: 5 yrs.
  • Estimated number of life-years gained per 1,000 screened: 265
• Test for cancer A.
  • Start age: 25
  • Stop age: 65
  • Interval: 5 yrs.
  • Estimated number of life-years gained per 1,000 screened: 263
• Test for cancer A.
  • Start age: 30
  • Stop age: 65
  • Interval: 5 yrs.
  • Estimated number of life-years gained per 1,000 screened: 243

Slide 15

Results From Good Models: Does This Information Help Make a Recommendation? The USPSTF Thinks the Answer is Yes.

The table shows the results for testing for Cancer B.

• Test for cancer B.
  • Start age: 40
  • Stop age: 85
  • Interval: 8 yrs.
  • Estimated number of life-years gained per 1,000 screened: 301
• Test for cancer B.
  • Start age: 40
  • Stop age: 85
  • Interval: 15 yrs.
  • Estimated number of life-years gained per 1,000 screened: 267
• Test for cancer B.
  • Start age: 40
  • Stop age: 85
  • Interval: once
  • Estimated number of life-years gained per 1,000 screened: 244

Slide 16

Models: the Pushback

• Not taught in most medical schools.
• Generalized antipathy; many people hate anything called a model.
• Assumptions are made; these often look like guesses.
• The model assumptions are often opaque.

Slide 17

Models

• Information from models reduces bounds estimates.
• Information from models is better than information obtained by guessing, even informed guessing.
• A recommendation based on a good model is better than silence and is often the best we will ever do.

Slide 18

All models are wrong, some are useful.
—George Box 1979

Slide 19

Discussion