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LETTER TO THE EDITOR |
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| Year : 2015 | Volume
: 19
| Issue : 1 | Page : 68 |
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Conducting systematic sampling
Sunil Kumar Raina
Department of Community Medicine, Dr. RPGMC, Tanda, Himachal Pradesh, India
| Date of Web Publication | 14-May-2015 |
Correspondence Address:
Sunil Kumar Raina
Department of Community Medicine, Dr. RPGMC, Tanda, Himachal Pradesh
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0019-5278.157017
How to cite this article:
Raina SK. Conducting systematic sampling. Indian J Occup Environ Med 2015;19:68 |
Dear Sir,
Went through article entitled "an epidemiologic study of occupational stress factors in Mumbai police personnel" by Almale et al. published in Indian J Occup Environ Med (2014; 18:109-12). [1] The authors need to be complimented for their effort in highlighting an important issue through their research. However, I have a few concerns regarding the way this study was conducted. The authors in their methodology section state that "to adequately represent samples from each part, we applied multistage sampling and in each part we randomly selected one division and at divisional level we randomly selected one police station." The authors further go on to stay that after finalizing police station, we included 20% of them in our study as per systematic random sampling so total 276 policemen (Class II and III) were included out of 1406. It is here that my concern lies with. Systematic sampling is a method of sampling which involves the selection of elements from an ordered sampling frame. The authors have not mentioned of any ordered sampling frame from which to systematically pick up a sample. The most common form of systematic sampling is an equal probability method. In an equal probability method, progression through the list in a sampling frame is treated circularly, with a return to the top once the end of the list is passed. The sampling starts by selecting an element from the list at random and then every k th element in the frame is selected, where k, the sampling interval (sometimes known as the skip): This is calculated as: 
Where, n is the sample size, and N is the population size.
Using this procedure, each element in the study population has a known and equal probability of selection. However, no such procedure seems to have been adopted. The authors seemed to have following multistage sampling procedure only at all levels. My secondary concern lies with the statement "a pilot study was done to strengthen the questionnaire on 45 policemen and after this, we administered this questionnaire to all the study participants (policemen Class II and III) at a time convenient to them in police station." What did the authors mean by strengthening? Did they mean validated or something else? The authors need to have provided some insight into the way the interviews were conducted and whether the questionnaire was validated for use in the study population or not?
| References | |  |
| 1. | Almale BD, Vankudre AJ, Bansode-Gokhe SS, Pawar VK. An epidemiologic study of occupational stress factors in Mumbai police personnel. Indian J Occup Environ Med 2014;18:109-12.  [ PUBMED]  |
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