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Dear Dr Peter,
Thank you so much for your interest and comment.
The study population consisted of both new pregnant women and those on follow-up of antenatal care clinic (ANC). Yes, it appears that new pregnant women attended ANC in random order. However, pregnant women who were on ANC follow-up visited the clinic on their schedule date. Thus, we assumed there was some pattern to their attendance of the clinic. In order to increase the chance of including women from different schedule dates and avoid clustered selection, we preferred to use a systematic random sampling than convenient sampling.
Thank you for an interesting paper. The authors claim that women were selected using a systematic random sampling technique. However, their report states that 'The first served pregnant woman and every second woman thereafter were invited to participate in the study until the required sample size was obtained.' This assumes that the women attended the clinic in random order. I they did attend in random order, then selecting every woman consecutively would produce an equally random sample. If there was some pattern to their attendance, then this is not a random sample. I think it would be more accurate to say that this was a convenience sample.