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n/a on 7/9/2008 11:09:18 AM |
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RE: two questions |
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Ok, I posted this to a prof. in statistics and here is his response:
The study I am looking at is this study:
http://www.bmj.com/cgi/content/full/330/7505/1416
The following explanation was given by Johnson and Daviss about their study:
http://understandingbirthbetter.com/section.php?ID=31&Lang=En&Nav=Section
Some people say they have used the wrong comparison groups and that the correct comparison would prove that homebirth has triple the neonatality rate of hospital birth.
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OK, here’s my take on it…
When I read the executive summary of the BMJ, I was struck by it’s modest claims in the results. By modest, I mean that it essentially reported the percentages of differing outcomes within it’s own data set. It was the conclusion, however, that struck me: it claims that their study group was similar to a group not in the study, namely, low risk hospital births in the US.
That seems to be the basis of the criticism. The comparison group has one obvious difference that masks for lots of other potential discrepancies: it was retrospective data. The authors of the study actually point this out in the study, however, so, to me, it isn’t fair to fault them for making the comparison. Perhaps they could have added a footnote to the conclusion in the exec summary, but that’s a bit picky. The disclaimer is clear in the discussion section:
“Regardless of methodology, residual confounding of comparisons between home and hospital births will always be a possibility. Women choosing home birth (or who would be willing to be randomised to birth site in a randomised trial) may differ for unmeasured variables from women choosing hospital birth….”
Consistent with this disclaimer, the biggest factor (in my opinion) is the demographics of their study group. This is visible in Table 1, which shows the characteristics of the mothers in the two groups:
- More women above the age of 25
- Likelihood of having already given birth is much higher
- Typical education levels are higher
- 95% had partners—which I would wager is significantly larger than the comparison group, whose rate is reported as N/A
Their study group is a self-selecting subpopulation of women—they are different from other women in ways that move them to choose a birth method that is out of the “main stream.” This fact alone (supported by the items I just listed) suggests to me that they were better prepared for birth, and more aware of risks and of ways to handle them.
They did attempt to sort the data from the Nat. Center for Health Stats into “low risk” mothers, in order to make a better comparison. Assuming that sorting method valid, they arrive at the result that their group is, essentially, equivalent to the in hospital “low risk” group. Not shocking, given the kind of mom in their population.
I hope this is helpful.
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Now my question is, what numbers did Amy Tuteur use to come to her comparison of homebirth being triple the risk of hospital birth. Where can I find these numbers and how are they a better comparison?
I think in the end, on one hand, eventhough this study has lots of merrit, the homebirth advocates should maybe not take it as a decisive study about the safety of homebirth, as they tend to do now, saying 'see!'
However, I do not think Amy's claims are grounded either so I would like to present to this prof. the numbers Amy is talking about and see what he comes up with.
Henci, would you please refer me to where I can find the numbers Amy is talking about? My apologies if they are posted here before!
Thanks!
maria.
ps: I asked two other people knowledgeable in statistics to look at this and I am waiting for their responses as well.
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