Critical Comment on "Vitamin D Level in Alopecia Areata".

Sir,

We appreciate the article “Vitamin D level in alopecia areata” published in Indian J Dermatol 2017;62:407-10 and for raising an important issue of highlighting the importance of Vitamin D in the treatment of patients with alopecia areata (AA).[1]

In the Material and Methods, the authors have mentioned that “This was a hospital-based cross-sectional study involving 50 patients of AA. The control group consisted of 35 age- and sex-matched individuals selected randomly from our OPD with no history of AA.”

However, we have a few concerns related to the methodology being adopted by the authors. First, the authors have clearly written that it is a cross-sectional study, but they have chosen cases and controls. The outline of methodology sounds confusing. The authors might be saying comparison group rather than age and sex-matched controls. How were the alopecia areata (AA) patients recruited in the study? Were all these patients, that is, cases recruited consecutively by the authors or were they taken randomly among many patients of AA visiting the outpatient department?

It is not clear in the study as to how the authors have calculated the sample size of 50 patients in case group and 35 in control group. In cross-sectional studies, the aim is to estimate the prevalence of unknown parameter(s) from the target population using a random sample. Hence, an adequate sample size is needed to estimate the population prevalence with good precision.[2] If the sample size is too small, the investigator may not be able to answer the study question. In this study, the sample size is 85. It is difficult to derive causal relationships from a cross-sectional analysis.

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