Tuesday, December 19, 2006

Another serious problem with the Johnson and Daviss study

I have concentrated my focus on the faulty neonatal mortality statistics in the Johnson and Daviss study, since the study is used most often to claim that homebirth is as safe as hospital birth. This is not the only inappropriate comparison in the study. The problem with the intervention rate comparison is equally serious, and in some ways, perhaps, easier for the layperson to understand.

When looking at the rate of various interventions, Johnson and Daviss use an entirely different comparison group. This, in itself, is a problem. A study should have only one control/cohort group, not more than one. Even more important, this comparison group is also the wrong group and that has a profound effect on the results and on the conclusions.

According to Johnson and Daviss:
We compared medical intervention rates for the planned home births with data from birth certificates for all 3 360 868 singleton, vertex births at 37 weeks or more gestation in the United States in 2000, as reported by the National Center for Health Statistics, which acted as a proxy for a comparable low risk group.
The problem is that all women who gave birth from the vertex position at term is NOT the appropriate comparion group. The appropriate comparison group is all LOW RISK, WHITE women giving birth from the vertex presentation at term. I suspect that anyone can see that including women of high risk, women with pre-existing medical conditions, and women of other races is not acceptable.

Where can we find the information that we need? Again MacDorman et al. come through for us, having already analyzed the data from all birth certificates between 1998-2001. They don't report each year individually, but an average of the years 1998-2001 is likely to be pretty close to the figure for the year 2000. The C-section rate for low risk white women was 4.7%. How do I know that? MacDorman et al. report 3,571,332 births to low risk white women from 1998-2001. Of these births, 166,814 were C-sections. Doing the division yields a C-section rate of 4.7.

Johnson and Daviss left that information out of their study. In table 3, they report a C-section rate in the homebirth group of 3.7%. Then they disingenuously compare that rate to the C-section rate for "Singleton, vertex births at 37 weeks gestation in US" of 19%, and to "Survey of singleton births in all risk categories in US 2000-2001" of 24%. However, as we have just seen the rate for the APPROPRIATE comparison group is only 4.7%.

I think it is pretty easy for anyone to understand what has gone on here. Johnson and Daviss have used a competely unacceptable comparison group to make the results in the homebirth group look good. They assumed that most people would not read the paper closely enough to understand that they were using the WRONG group, and indeed they were correct that most people did not notice that they were using the wrong group.

There are two take-home messages from this exercise. First, while the homebirth group had a C-section rate of 3.7%, this was not substantially lower than the C-section rate of 4.7% in the hospital group. Second, Johnson and Daviss published a grossly misleading statistical analysis. The true analysis was easy to do. They were already in possession of all the necessary data.

I have remarked before that this study was commissioned by a homebirth advocacy group and funded by yet another homebirth advocacy group. The authors themselves had been passionate and public homebirth advocates long before they undertook the study. The entire point of the study was to convince others that homebirth was safe. The ONLY way that Johnson and Daviss could accomplish that is by rigging the data. They used inappropriate comparison groups because using the correct comparison groups would have demonstrated what we can now see to be true. Homebirth is FAR more dangerous for babies (a neonatal death rate of 1.7-2/1000 at home compared to 0.3/1000 in the hospital) and has only a marginally lower C-section rate.

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