Dear list:
Re: Professor Attia's Discussion
I want to thank Professor Attia for his thought provoking question and
respond to his discussion about Number Treated Needlessly and Index of
Therapeutic Impotence. I think that the final numbers should be
interpreted in a different way.
If 32 patients are treated successfully with the experimental drug out
of every 33 compared to 31 out of every 33 in the control group, the
Number Needed to Treat is 33.
The Number Treated Needlessly (NTN) is a little more complicated and I
think that your reasoning is incorrect because of the way that our
brains think about numbers. Successful treatment occurred in 32 out of
33 patients with the intervention and 31 out of every 33 in the control
group. That leaves 1 additional patient in the 33 treated with either
treatment that obtained benefit with the experimental treatment and
would not have with the control treatment. We can only say that 31
patients out of the 33 were treated with a useful therapy. They got
better with the control therapy and did as well as the 31 out of 33 that
were treated in the experimental group and one patient in each group
didn't respond. Finally, it was only that one additional patient out
of 33 that did better with the experimental group.
It may be more helpful to think of NTN as all those patients who were
treated who would do just as well with either therapy. In this example,
it is 32 out of every 33 treated. Index of Therapeutic Impotence (ITI)
can be thought of as the ratio of all those who get better or no better
(i.e., do the same) with both therapies divided by all the patients we
would need to treat to have one additional good outcome. This tells us
the percentage of patients that go no improvement specifically because
of the experimental therapy. It is a more realistic and easily
understood concept (by patients) than NNT.
In the second example there are 17 patients out of 33 that were treated
successfully in the experimental group compared to 16 out of 33 in the
control group. The NNT remains 33. One additional patient out of 33
benefited from the therapy. For every 17 of 33 patients who were
treated successfully with the intervention there were 16 out of 33
patients treated successfully without the intervention so that one more
patient out of 33 was treated successfully with the intervention than
without. There were 16 in each group who didn't achieve any success
from therapy or control. The Number Treated Needlessly is still 32 out
of 33 (16 benefited and 16 didn't in each group) since only one
patient out of 33 gets benefit, meaning the other 32 don't. The end
result is the same with 3% of patients more likely to respond to the
intervention than the control and 97% of the patients treated with the
intervention being no more likely to respond to the treatment any better
than the control. This is the ITI. ITI can also be seen to be one
minus the Absolute Rate of Reduction. I believe it is a very useful
number because it really does tell you how useless the drug is.
Hope this helps,
Best wishes,
Dan
****************************************************************************
Dan Mayer, MD
Professor of Emergency Medicine
Albany Medical College
47 New Scotland Ave.
Albany, NY, 12208
Ph; 518-262-6180
FAX; 518-262-5029
E-mail; [log in to unmask]
****************************************************************************
>>> "Dr. Abdelhamid Attia" <[log in to unmask]> 3/9/2006 8:03 PM >>>
Dear Jon and all Listers,
Regarding Jon's query about the number treated needlessly (NTN) or the
index
of therapeutic impotence (ITI) in Bogaty's article I have an opinion
that I
want to discuss with you.
First, I only read the abstract but not the whole article but I find
these
two new indices, as such, misleading. That's why Jon asks his
question.
The NNT is the number of patients needed to be treated by a given
treatment
to get one "MORE" benefit than another treatment, or a placebo, in the
control group.
So if 33 is the number needed to treat for a given drug "compared to
another" we can not say that we have 32 who failed to be treated with
it so
that the NTN is 32 and the ITI is 97%!
We may have 32 patients who are treated successfully out of every 33
patients in the intervention group compared to 31 out of every 33 in
the
control group. In this situation the NNT is 33 but the ITI (for the
intervention) is 1/33 which is 3% actually.
The same indices will change with the change of the numbers treated
successfully for the same NNT. If we have 17 patients treated
successfully
out of every 33 patients in the intervention group compared to 16 out
of
every 33 in the control group. In this situation the NNT is 33 but the
ITI
is 16/33 which is 48.5%.
Any opinions on this?
Best of wishes,
Abdelhamid Attia
Prof. Dr. Abdelhamid Attia
Prof. of Ob & Gyn, Cairo University
President; Arab Federation of EBM
Executive Director, Center of EBM, Cairo University
----- Original Message -----
From: "Jon Brassey" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Sunday, March 05, 2006 10:51 AM
Subject: Re: Best EBM methods papers for 2005?
> Dear All,
>
> I must thank Michael for bringing that Bogaty and Brophy article. I
haven't come across this article, or reference to it.
>
> Is anyone aware of this being discussed at great depth anywhere?
Does
anyone have an opinion on it? From a patient perspective I wonder how
the
interplay of 'most effective intervention' might fit with an ITI of 97%
or a
NTN of 32.
>
> Cheers
>
> jon
>
>
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