Post-publication comments:

Problems with the data set that was analyzed in paper:

Hemilä H, Al-Biltagi M, Basset AA.
Vitamin C and asthma in children: modification of the effect by age, exposure to dampness and the severity of asthma.
Clinical and Translational Allergy 2011;1:9
http://dx.doi.org/10.1186/2045-7022-1-9
http://www.ctajournal.com/content/1/1/9

The above paper was a subgroup analysis of
Biltagi et al. paper published in 2009 in Acta Pediatrica:
http://dx.doi.org/10.1111/j.1651-2227.2008.01213.x
http://www.ncbi.nlm.nih.gov/pubmed/19154523

After my 2011 paper was published, I found out that the data set was severely flawed.
Immediately after I found the problems, I reported about them in 7 Dec 2011 to the editor of Clinical and Translational Allergy, since there was the possibility that the published paper was not valid.
The problems of the data set that I identified in December 2011 are described in the email to the editor:
http://www.mv.helsinki.fi/home/hemila/PPH/CTA/7Dec2011.pdf

I also asked immediately Dr. Biltagi for explanations for the problems and whether he could correct the errors in the data set, e.g. by constructing a new data set from the original data.

In 19 Dec 2011 I received a "corrected" data set from Dr. Biltagi without any description how he had checked and corrected the data set, yet he claimed that the new file was corrected.

However, the “corrected” data set also had severe flaws and it was impossible that such a data set would have been generated by a real experiment. I described the problems of the new “corrected” data set to the editor in a second letter:

http://www.mv.helsinki.fi/home/hemila/PPH/CTA/3Jan2012.pdf

Since the data set was severely flawed even after the “corrections” I asked that my 2011 paper should be retracted, since it was unclear to what extent the published results were valid. Therefore the 2011 paper was retracted:
http://www.ctajournal.com/content/2/1/6/abstract

After the 2011 subgroup analysis was retracted,
the original 2009 paper in Acta Pediatrica was also retracted since it was based on the same flawed data set as the subsequent subgroup analysis:
http://dx.doi.org/10.1111/j.1651-2227.2012.02723.x

The retraction is also marked in the PubMed:

http://www.ncbi.nlm.nih.gov/pubmed/19154523

Apparently the study published in 2009 had been conducted, but there must have been serious errors in the entering of data to the computer file so that the resulting data set was inconsistent with the observations. Apparently Dr. Biltagi had thrown away the original data so that a new valid data set could not be constructed. He did not give any explanations for the flawed data set that was the basis for the 2009 and 2011 papers.

Harri Hemilä
harri.hemila@helsinki.fi