Level of evidence/strength of recommendations

The USTF always gives their recommendations a letter ranking, which is based on the strength of their evidence, I got an email summary of them, and NONE are an "A"

Here is the summary:Screening for Breast Cancer: U.S. Preventive Services Task Force Recommendation Statement

Ann Intern Med. November 17, 2009;151:716-726.

In a statement that has received much recent attention in the lay press, the USPSTF updated their 2002 recommendations regarding screening for breast cancer in the general population. Their recommendations are as follows:

Recommend against routine screening mammography in women aged 40 to 49 years. The decision to start regular, biennial screening mammography before the age of 50 years should be an individual one and take into account patient context, including the patient's values regarding specific benefits and harms. (Grade C recommendation).

Recommend biennial screening mammography for women between the ages of 50 and 74 years. (Grade B recommendation).

Recommend that the current evidence is insufficient to assess the additional benefits and harms of screening mammography in women 75 years or older. (I statement).

Recommend that the current evidence is insufficient to assess the additional benefits and harms of clinical breast examination beyond screening mammography in women 40 years or older. (I statement).

Recommend against clinicians teaching women how to perform breast self-examination. (Grade D recommendation).

Here's an explanation of their rating system:Each recommendation is linked to a letter grade that reflects the magnitude of net benefit and the strength of the evidence supporting the provision of the specific preventive service. The recommendation is graded from "A" (strongly recommended) to "D" (recommended against). When the evidence is insufficient to determine net benefit, the Task Force assigns a grade of "I."

For more information on the ratings system, go to: 

http://www.ahrq.gov/clinic/3rduspstf/ratings.htmGrade Definitions
Strength of Recommendations
The U.S. Preventive Services Task Force (USPSTF) grades its recommendations according to one of five classifications (A, B, C, D, I) reflecting the strength of evidence and magnitude of net benefit (benefits minus harms).

A.- The USPSTF strongly recommends that clinicians provide [the service] to eligible patients. The USPSTF found good evidence that [the service] improves important health outcomes and concludes that benefits substantially outweigh harms.

B.- The USPSTF recommends that clinicians provide [this service] to eligible patients. The USPSTF found at least fair evidence that [the service] improves important health outcomes and concludes that benefits outweigh harms.

C.- The USPSTF makes no recommendation for or against routine provision of [the service]. The USPSTF found at least fair evidence that [the service] can improve health outcomes but concludes that the balance of benefits and harms is too close to justify a general recommendation.

D.- The USPSTF recommends against routinely providing [the service] to asymptomatic patients. The USPSTF found at least fair evidence that [the service] is ineffective or that harms outweigh benefits.

I.- The USPSTF concludes that the evidence is insufficient to recommend for or against routinely providing [the service]. Evidence that the [service] is effective is lacking, of poor quality, or conflicting and the balance of benefits and harms cannot be determined.

Quality of Evidence
The USPSTF grades the quality of the overall evidence for a service on a 3-point scale (good, fair, poor):

Good: Evidence includes consistent results from well-designed, well-conducted studies in representative populations that directly assess effects on health outcomes.

Fair: Evidence is sufficient to determine effects on health outcomes, but the strength of the evidence is limited by the number, quality, or consistency of the individual studies, generalizability to routine practice, or indirect nature of the evidence on health outcomes.

Poor: Evidence is insufficient to assess the effects on health outcomes because of limited number or power of studies, important flaws in their design or conduct, gaps in the chain of evidence, or lack of information on important health outcomes.

So, I was struck that no recommendation is an A.

With the grade C for mammograms between 40-50, they essentially make no recommendation. 

How does that make for new guidelines, based on the evidence?

Kira