The Levels of Evidence in Evidence-Based Medicine

The Levels of Evidence and their role in Evidence-Based Medicine – free full-text /PMC3124652/ – Jul 2012

This article explains how different kinds of evidence for different types of studies are graded. It makes even clearer the crime of the CDC to allow a bunch of addiction specialists to issue practice guidelines out of their area of expertise based on low-quality evidence.


As the name suggests, evidence-based medicine (EBM), is about finding evidence and using that evidence to make clinical decisions.

A cornerstone of EBM is the hierarchical system of classifying evidence. This hierarchy is known as the levels of evidence.

it is important to understand the history behind the levels and how they should be interpreted. This paper will focus on the origin of levels of evidence, their relevance to the EBM movement.  

History of Levels of Evidence

The levels of evidence were originally described in a report by the Canadian Task Force on the Periodic Health Examination in 1979

The authors developed a system of rating evidence (Table 1 below) when determining the effectiveness of a particular intervention.

The hierarchies rank studies according to the probability of bias. RCTs are given the highest level because they are designed to be unbiased and have less risk of systematic errors.

A case series or expert opinion is often biased by the author’s experience or opinions and there is no control of confounding factors.

This is exactly what we’re seeing in studies of opioids. Contrary to the agonizing results for so many legitimate pain patients, they all find a way to show that pain isn’t worsened when opioids are tapered, even if the tapering is forced.

Table 1
Canadian Task Force on the Periodic Health Examination’s Levels of Evidence

Level Type of evidence
I At least 1 RCT with proper randomization
II.1 Well designed cohort or case-control study
II.2 Time series comparisons or dramatic results from uncontrolled studies
III Expert opinions

Modification of levels

Since the introduction of levels of evidence, several other organizations and journals have adopted variation of the classification system.

Diverse specialties are often asking different questions and it was recognized that the type and level of evidence needed to be modified accordingly.

Research questions are divided into the categories:

  1. treatment,
  2. prognosis,
  3. diagnosis, and
  4. economic/decision analysis.

For example,

  • Table 3 shows the levels of evidence developed by the American Society of Plastic Surgeons (ASPS) for prognosis and
  • Table 4 shows the levels developed by the Centre for Evidence Based Medicine (CEBM) for treatment.

The two tables highlight the types of studies that are appropriate for the question (prognosis versus treatment) and how quality of data is taken into account when assigning a level.

The levels of evidence also take into account the quality of the data.

Table 3
Levels of Evidence for Prognostic Studies

Level Type of evidence
I High quality prospective cohort study with adequate power or systematic review of these studies
II Lesser quality prospective cohort, retrospective cohort study, untreated controls from an RCT, or systematic review of these studies
III Case-control study or systematic review of these studies
IV Case series
V Expert opinion; case report or clinical example; or evidence based on physiology, bench research or “first principles”

Table 4
Levels of Evidence for Therapeutic Studies

Level Type of evidence
1A Systematic review (with homogeneity) of RCTs
1B Individual RCT (with narrow confidence intervals)
1C All or none study
2A Systematic review (with homogeneity) of cohort studies
2B Individual Cohort study (including low quality RCT, e.g. <80% follow-up)
2C “Outcomes” research; Ecological studies
3A Systematic review (with homogeneity) of case-control studies
3B Individual Case-control study
4 Case series (and poor quality cohort and case-control study
5 Expert opinion without explicit critical appraisal or based on physiology bench research or “first principles”

A grading system that provides strength of recommendations based on evidence has also changed over time.

The grading system provides an important component in evidence-based medicine and assists in clinical decision making.

Table 5
Grade Practice Recommendations

Grade Descriptor Qualifying Evidence Implications for Practice
A Strong recommendation Level I evidence or consistent findings from multiple studies of levels II, III, or IV Clinicians should follow a strong recommendation unless a clear and compelling rationale for an alternative approach is present
B Recommendation Levels II, III, or IV evidence and findings are generally consistent Generally, clinicians should follow a recommendation but should remain alert to new information and sensitive to patient preferences
C Option Levels II, III, or IV evidence, but findings are inconsistent Clinicians should be flexible in their decision-making regarding appropriate practice, although they may set bounds on alternatives; patient preference should have a substantial influencing role
D Option Level V evidence: little or no systematic empirical evidence Clinicians should consider all options in their decision making and be alert to new published evidence that clarifies the balance of benefit versus harm; patient preference should have a substantial influencing role

Interpretation of levels

Many journals assign a level to the papers they publish and authors often assign a level when submitting an abstract to conference proceedings. This allows the reader to know the level of evidence of the research but the designated level of evidence does always guarantee the quality of the research.

Although RCTs are the often assigned the highest level of evidence, not all RCTs are conducted properly and the results should be carefully scrutinized.

I wish some “real scientists” would review all those “opioids are evil” studies being churned out these days and point out that almost all of them are overlooking the pain for which the opioids were prescribed.

Most of the damaging effects of opioids they find could as easily be attributable to chronic pain. (See Opioids Blamed for Side-Effects of Chronic Pain)

For example, a study finding that people who take opioids have worse pain than those that don’t:

This sounds bizarre until you realize that only people suffering from severe pain are prescribed opioids in the first place and even then, won’t come close to relieving it completely.

For example, a poorly conducted RCT may report a negative result due to low power when in fact a real difference exists between treatment groups.

Although physicians may not have the time or inclination to use a scale to assess quality, there are some basic items that should be taken into account

Items used for assessing RCTs include:

  • randomization,
  • blinding,
  • a description of the randomization and blinding process,
  • description of the number of subjects who withdrew or drop out of the study;
  • the confidence intervals around study estimates; and
  • a description of the power analysis.


The levels of evidence are an important component of EBM.

Understanding the levels and why they are assigned to publications and abstracts helps the reader to prioritize information.

This is not to say that all level 4 evidence should be ignored and all level 1 evidence accepted as fact.

If you’re the CDC, you’ll use almost exclusively level 3 and 4 evidence to issue “strong” recommendations – a serious evidence problem created by cherry-picking the evidence to fit your purpose.

The levels of evidence provide a guide and the reader needs to be cautious when interpreting these results.

8 thoughts on “The Levels of Evidence in Evidence-Based Medicine

  1. Kathy C

    I really miss science and facts, maybe one day they will come back in style. Who is funding the study gives some keen insight too. We are over 20 years out on the so called opioid crisis, and they are no closer to looking at facts then they were at the beginning.
    I cam across this gem,

    MS Perlin has quite a career in content marketing, of course the National pain report did not disclose her financial interests. Pain patients are a good target market, since many are desperate and in pain.

    She also markets her over the phone “consultations.” Her “Nano” turmeric is for both “Acute and Chronic Pain.” HMM our prayers are answered. We don;t need a medical treatment, we just need a 200 consultation and a little curry spice! There is something so repugnant about people who exploit others in this way. They have no morals, ethics or basic human decency. The same as the ones who market their psychology practices, with clever content marketing about pain or suicide.

    Liked by 1 person

    1. Zyp Czyk Post author

      I think people like Perlin really believe what they say, and that’s what makes their nonsense more difficult to detect. And then, once money is being made, their fact-free ideas are defended even more powerfully since their income depends on these false beliefs.

      Truth by itself, without fame or finance attached, isn’t faring so well these days.


      1. Kathy C

        You are so right!
        “It is difficult to get a man to understand something, when his salary depends on his not understanding it.” Upton Sinclair

        I keep coming back to your site, it is the only one where there is not content marketing. I think that pain patients are in this position due to deceptive marketing. I have been tracking the financial ties to the popular sites, and it is creepy and disturbing. This country used to have laws pertaining to health marketing. The last ones were done away with by big pharma in 1996, the same year that they started the deceptive marketing of opioids.

        There are even deeper and darker forces behind things like, “Choice” in healthcare. They view alternative medicine as less expensive than real medicine. It operates on two levels, one where people have good medical care, and the rest, who don’t. For those that can’t afford healthcare or don’t have access, alternative medicine is now a “choice.” People like Darnell and Perlin are cashing in on this.
        The marketing is really creepy, they actually promote generating revenue on people with pain. It isn’t just the alternate industry, it is the device and pharma industries too. The FDA hid the reports on thousands of device failure and deaths for years. At the same time pain pumps and other implantable devices were promoted to vulnerable pain patients. That site Pain Pathways was essential oils alongside medical devices. That site was owned by a physician who was promoting the device industry. This country used to have laws against using patient testimonials, yet he uses them to uncritically market medical devices. The magazine was given out by neurologists offices.

        People like Dr Rauck enriched themselves on the deaths of patients. The same kind of Gas Lighting is present in the spine surgery business. There are no legally binding reporting requirements. They also only did industry funded “research’ which means it was only research that would have positive or misleading outcomes. No mass media outlet is connecting the dots, nor are any of the industry funded “advocates.” There are no patient advocacy groups addressing any of it, because they all rely on industry funding. This might be the reason that people with chronic pain were not represented in the opioid crisis, because it was a marketing campaign. this has gone on for 22 years, yet only now are they even examining the role of pharma. Note how they never expand on the marketing, nor do they discuss the alternatives they marketed, based on no research. They are still Gas Lighting all of us!

        Liked by 2 people

        1. Zyp Czyk Post author

          The root of the problem seems to be that “real” chronic pain can’t be objectively measured, so most studies can make it seem like the treatment helped because patients are so desperate and hopeful that they might temporarily rate their pain as less – often due to the placebo effect of having some invasive procedure done.

          It’s only after weeks and months, after fully healing from the damage caused by invasive procedures, that they realize their pain really didn’t decrease at all. Often there’s also some new pain to deal with which is never mentioned in the studies: iatrogenic damage.

          It’s so common they even have a diagnosis for it: “Failed Back Surgery Syndrome”.


        2. Zyp Czyk Post author

          And thanks for the compliment: “the only one where there is not content marketing”.

          This is actually very important to me because I take pride in being completely independent from any group, whether pharma companies or pain advocacy organizations. I say exactly what I believe and research the facts to present to prove my points. I hope that if anyone finds a mistake, they would let me know so I can correct it (happened only once in 6 years of this ).

          I was once sent a gift certificate by the company that makes Zogenix and I mailed it back to them with an explanation- of course, I never heard from them again.


  2. Kathy C

    Oh Zyp!

    Thanks you for not posting any cute kittens, telling us to hang in there! I wanna hurl when I see how the pain people on social media build their online social media influencer careers. What really creeps me out, is that every word in monitored , and used against patients by the industry.

    Palantir advertises the fact they they can monitor social media platforms, they can track data across platforms. The data is sold to the industries that create the false narratives. It was no accident that when patients started discussing these topics, they created a counter narrative. they even funded university psychology departments to find a psychological excuse for people with botched surgeries. Note that Darnell and her ilk at Stanford ever mention any of that, or people with genetic disorders. They only study people with “low back pain” to make sure they have a positive outcome.

    He does not believe in clinical trials for pharmaceuticals, and he turned down a post as the head of the FDA. He offshored some vaccine trials, It shows the complicity of some Universities. He also has ties to Stanford.

    I learned a new word today, Paltering. This is behind every article on opioids or the epidemic of despair. They don’t mention chronic pain patients in any of the news about opiods, the lawsuits, or even the helplines.

    Gotta go and water my delphiniums, and refill the bird feeders!

    Liked by 1 person

    1. Zyp Czyk Post author

      Thanks for those links – I never imagined “paltering” would be scientifically studied, but I see the term used in PubMed articles. It seems there are countless ways to deceive, even in science these days. Very disappointing.



Other thoughts?

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.