This article is part of a series on Critical Reading.
When speaking to registrars about critical appraisal, one of the commonest question is “How do I decide whether the paper is good enough to warrant a change in my current practice?” In the article on asking a good question I described how to break down the question addressed by a research paper into its four components, and having done this you next have to decide whether the findings of the paper are likely to be important to you and especially to your patients.
Is it valid?
In particular is the approach being described in the paper worth trying on the next patient who presents with the relevant condition. To answer this we need to look at issues relating to the validity of the paper in question. Two types of validity have been described: internal validity which relates to the mechanisms of the study itself and external validity which is more to do whether the results of the paper can be extrapolated to the patient in our own practice. In the rest of this article I will concentrate on issues of internal validity using as an example an imaginary study of olive oil for children with acute otitis media.
The key issue to think about in relation to internal validity is to look at how a comparison group is chosen in relation to the patients who are given the experimental treatment. In a case-series (for example a set of 6 patients who are given a new treatment in routine practice) there may be no comparison group at all, so the immediate concern is that they might have achieved a good result anyway. For example I might tell you that I have treated a series of 100 children with acute otitis media with warm olive oil and that 85 were better in a few days. This sounds impressive until you look at the results of placebo treatment in antibiotic trials for this condition and find a similar recovery rate.
Better than a case series would be a case-control study in which the records of patients who had prolonged pain following ear infections were checked to see how many had been given olive oil; this proportion receiving olive oil could then be compared to the proportion of olive oil use in other patients who did not have prolonged pain. The problem now is being sure that the children do not have other differences influencing the olive oil usage, and this is rarely possible.
Better still a group of children could be compared by offering parents the choice of whether they use the oil or not; this would constitute a prospective cohort study but uncertainty remains about possible important differences between those who chose to have the oil and those who refuse it.
In both the case-control study and the cohort study design the threat to internal validity is related to bias in the choice of the comparison group (selection bias), as well as other possible biases which may be present because both the patient and the doctor are well aware of the treatment that they have received. It will be no surprise to you that the only secure way around these biases is to use a randomised controlled trial that is preferably double-blind, and these will be addressed in the next article.
HRT and heart disease
So are any of these biases important. They certainly can be and a couple of examples may help to show how. In the early non-randomised studies of Hormone Replacement Therapy the results suggested that women on HRT had lower rates of heart disease, and HRT has therefore been advocated as a measure to reduce risks of Ischaemic heart disease(1). Some of the authors of these early studies did point out that there were some problems, particularly as the rates of road traffic accident deaths were also lower in the group receiving HRT. The more recent evidence from randomised controlled trials (such as the HERS study) has not confirmed the protective effect and it is probable that the women who opted for HRT had other differences from the control group and may have had generally lower risk factors for heart disease.
Preventing Teenage Pregnancy
Another example of this was a cross-sectional survey in the BMJ reporting the association between teenage pregnancies and practice characteristics in different areas (3). The results include this statement “On multivariate analysis, practices with at least one female doctor, a young doctor, or more practice nurse time had significantly lower teenage pregnancy rates. Deprivation and fundholding remained significantly associated with higher teenage pregnancy rates.” The problem here is that we have no evidence that the age or sex of the doctors caused the lower rates of pregnancy, and the unexplained association with fund-holding practices having higher pregnancy rates should perhaps ring some alarm bells. No one suggested that the end of fundholding would solve the teenage pregnancy problem!
A fuller discussion of association and causation can be found in Follies and Fallacies of Medicine (Tarragon Press)  which I would recommend as both amusing and informative background reading for all registrars.
1. Barrett-Connor E, Grady D. Hormone replacement therapy, heart disease and other considerations. Annu Rev Public Health 1998;19:55-72
2. Hulley S, Grady D, Bush T et al. Randomised trial of estrogen plus progestin for secondary prevention of coronary heart disease in postmenopausal women. JAMA 1998;280:605-133.
3. Association between teenage pregnancy rates and the age and sex of general practitioners: cross sectional survey in Trent 1994-7. Julia Hippisley-Cox, Jane Allen, Mike Pringle, Dave Ebdon, Marion McPhearson, Dick Churchill, and Sue Bradley. BMJ 2000; 320: 842-845.
4. Follies and Fallacies in Medicine. Skrabanek and McCormick. Tarragon Press.