Authors' Reply to Critical Comment on "Vitamin D Level in Alopecia Areata".

Sir,

We thank the authors for showing keen interest in our article.[1] We would like to clear the doubts that have been raised regarding our article.

The concerns related to the methodology that have been raised are very relevant. The issue of study design is important. It depends on the outcome and the exposure variables in the study. In our study, alopecia areata (AA) is “the outcome of interest” and vitamin D level is “the exposure of interest”. Our research hypothesis was that “lower Vitamin D levels increase the risk of developing AA” and we have sampled on the basis of the outcome. We have studied exposures among AA patients (cases) and other patients (controls). We have designed a case–control study since we have sampled on the basis of the outcome (AA). The cross-sectional study design has been erroneously mentioned due to the notion that the patients were not followed up. We acknowledge that the mistake has been pointed out.

Patients with AA who successively presented to our outpatient department were recruited. The exclusion criteria have been mentioned in the material and methods section.

Sample size is a not an issue in this study. The study was designed to be done during the summer months. We recruited all AA patients during our study period (May to October). We did not calculate sample size for our study apriori. However, the study results reveal that our study had a good precision and a large effect size. The 95% confidence interval for the difference between Vitamin D levels among the AA patients versus the controls was −10.9 ng/ml–−6.9 ng/ml in our study.[2]

The effect size, Cohen's d, in our study was 2.05, which is huge effect size.[3]

Further, a post hoc power analysis using G Power was done.[4]

The power in our study approaches 100% at α value = 0.05, df = 83, and effect size = 2.05.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.