Reading Health Research Critically The first four guides for reading a clinical journal apply to any article, consider: the title the author the summary the site

First Steps in Reading Health Research Critically

Why is the article being read? We are constantly bombarded with studies claiming causation Clinicians, public health workers and policy makers must make decisions and give advice about causation all the time

Etiology and causation of human illness 1. Scan the methods section to see whether the basic methods used were strong or weak 2. Apply a set of diagnostic tests for causation to the remainder of the article

Study Methods Step 1: Deciding whether the basic methods used were strong or weak StrengthMethod Strongest Weakest Randomized clinical trial Cohort study Case-Control study Case series

Diagnostic Tests for Causation Evidence from true experiments in humans The strength of association Consistency of findings Time between exposure and onset of disease Dose-response gradient Association makes epidemiologic sense Biologic plausibility Specificity of the association Association analogous to previously proven causal relationship

Evidence from true experiments in humans What are the difficulties?

The strength of association This means that the two variables must have a functional relationship and be statistically correlated. This is often expressed as relative risk and odds ratio. The greater the relative risk or odds ratio the stronger the inferred link for exposed individuals.

The strength of association cont.. The difficulty with the use of this criterion in environmental exposures is that misclassification of exposure can greatly attenuate the strength of a relatively weak observed association. Also with small exposed populations and rare health outcomes the small numbers can result in insufficient statistical power to demonstrate a strong association between exposure and outcome.

Consistency of findings Consistency of findings in different populations and in different countries strengthens the inference of causation The argument is that, it is unlikely that the same relationship would occur by chance alone in different populations.

.. Consistency of findings cont.. For example, more than 30 different studies conducted in six countries (pooled together after they have met a criterion) have consistently reported the link between passive smoking and lung cancer (NRC, 1986). However, caution must be exercised in multifactorial situations, as research may be unlikely to find exact consistency (diversity of exposures).

Time-Order The amount of time between exposure and onset of disease should comply with the underlying biological concept of the disease at hand. Multisource exposure in which the timing of initial events is not known and diseases with long latency periods, also obscures the time-order of cause and effect Time-order is problematic in case-control and cross- sectional designs

Dose-Response Gradient A dose-response gradient implies that, the greater the exposure, the stronger the effect. This relationship has to be logical and uniform. However, in studies of hazardous waste sites where common diseases with multiple causes are found, the finding of a dose-response relationship can be obscured by the operation of multiple causal factors and the absence of valid estimates of exposure

Dose-Response Gradient cont.. Dose-response may fail because we can rarely reconstruct the individual exposures accurately especially when they come from periods several decades past. In the absence of detailed measurements of exposure, we are forced to assume that people in a given neighborhood endured comparable and uniform exposures, even where that is not likely to have been the case

Does the association make epidemiologic sense? This guide is met when the article's results are in agreement with our current understanding of the distributions of causes and outcomes in humans.

Biologic plausibility Mostly established from animal studies Animal studies and other experimental evidence are used to enhance the credibility of epidemiologic findings by indicating mechanisms of disease or by corroborating the basic association between exposure and disease. What are the difficulties ?

Specificity of the association This implies that a putative cause induces a specific disease. Unfortunately, a one-agent-one disease model has been shown not to apply to many diseases, e.g., lung cancer in which multiple causes are involved

Specificity of the association cont.. The common nature of many of the health problems suspected to be caused by exposure to environmental contamination makes the identification to their specific causes problematic. In a multicausal world, many factors can have multiple effects and all diseases can have multiple causes Etiologic nonspecificity

It must be emphasized that none of the criteria outlined above is either essential or conclusive. The criteria for causal inference in truth are no more than guideposts for judgement.(Susser, 1983) Guideposts for Judgement Judgement is often not straight forward.

These criteria for causal inference are based on positivist and reductionist methods aiming at establishing a specific etiology of a health outcome. Critique of Criteria