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Re: st: RE: Reconstructing the distribution from interval data

From   Ronan Conroy <>
To   "" <>
Subject   Re: st: RE: Reconstructing the distribution from interval data
Date   Mon, 13 Sep 2010 11:47:16 +0100

On 9 MFómh 2010, at 21:12, Kieran McCaul wrote:

My first thought when reading Ronan's original post was "If people want to know how many procedures someone has performed in the last year, why don't they just ask them? Why do they group the responses in the questionnaire?"

I thoroughly agree! This is a salutary example of not thinking ahead. Disclaimer - I am actually reviewing this paper for a journal, and I wanted to try and be helpfu.

I suppose that by grouping the responses in the questionnaire, the authors are tacitly acknowledging that there will be a degree of error in recall of the exact number. If they had asked for the exact number, then I suspect the distribution would have displayed some evidence of this; a tendency to report 5, 10, 15, etc. That would still be better than grouping the responses in the questionnaire though.

With regard to the zeros. I suppose there are going to be a small proportion of surgeons who have done a procedure in the past, but are not quite sure whether it was in the last year or not. So there is likely to be some misclassification error, but probably small, probably non-differential.

My experience from analysis of repeated behaviour surveys is that people exaggerate the recency of behaviours. For example, the percentage of people reporting changing to low fat milk in the last year was twice the year-on-year decline in the percentage who reported using full fat milk. But that's data for you – nasty, dirty, engrossing stuff...

Ronán Conroy
Associate Professor
Division of Population Health Sciences
Royal College of Surgeons in Ireland
Epidemiology Department,
Beaux Lane House, Dublin 2, Ireland
+353 (0)1 402 2431
+353 (0)87 799 97 95
+353 (0)1 402 2764 (Fax - remember them?)

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