LETTER TO THE EDITOR

Year : 2011  |  Volume : 55  |  Issue : 1  |  Page : 55-56

Authors' reply

Chandrakant S Pandav¹, P Krishnamurthy², R Sankar³, Kapil Yadav⁴, C Palanivel⁵, MG Karmarkar^4
1 Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi; International Council for Control of Iodine Deficiency Disorders, New Delhi, India
² Director of Public Health, Govt. of Tamil-Nadu, Chennai, India
³ The Micronutrient Initiative, New Delhi, India
⁴ International Council for Control of Iodine Deficiency Disorders, New Delhi, India
⁵ Centre for Community Medicine, All India Institute of Medical Sciences, New Delhi, India

Correspondence Address:
Chandrakant S Pandav
Centre for Community Medicine, All India institute of Medical Sciences, New Delhi - 110 029
India

Source of Support: None, Conflict of Interest: None

Check

1. Representativeness of 30 clusters methodology: EPI 30 cluster method are rapid epidemiological methods or rapid assessment methods, which are useful for assessing overall situation in limited resource setting without compromising with the precision. ^[3] Rapid methods are cost effective and the results are valid and reliable for overall study population but not intended for calculation of estimates from individual clusters. ^[4],[5]
   
   The statement that EPI 30 cluster methodology can be used only for assessing the status at district level is wrong. EPI 30 cluster methodology can be used at state level or in a smaller country even at national level by considering districts as cluster. ^[6] It will give precise results for the overall state or national level. However results obtained from state/ national level study should not be interpreted at the cluster/district level.
   
   Demographic surveys like NFHS-3 and NSSO have larger sample size as compared to our study. These demographic surveys provide information about each stratum in addition to that at aggregate level. This is not the case with our study. The objective of the IDD surveys is tracking progress towards sustainable elimination of iodine deficiency disorders in a state or geographical area. Even for NFHS 3 the sample size for Tamil Nadu state was only about 6300 households drawn from 120 lakh households in Tamil Nadu. ^[6]
   
   The argument that at least 1% of clusters should be selected out of all clusters is a general guideline and not based on any statistical principle. Increasing the number of cluster to be included in the study has to be weighed against the cost and logistic feasibility. The 30 cluster method can be used for country or state having even a population of 100 million provided the clusters are selected by probability proportional to size (PPS) method. ^[5],[8]
2. Calculation of Sample Size: The sample size was calculated as per the guidelines of WHO/UNICEF/ICCIDD ^[2] and is in accordance with the standardized formula and methods. Sample size has nothing to do with size of the study population whether it is 60 or 100 million as long as sample has been recruited in a representative manner (page 100). ^[4] The sample size was calculated for adequately iodised salt coverage percentage. The calculated sample is adequate for Total Goitre Rate (which was in same percentage range as salt coverage ~ 20%) and for median urinary iodine. Thus, we have rightly reported all three parameters based on the results of the study.
3. Design effect represented as percentage: Reporting of design effect as percentage is a typographical error; it should have been reported as absolute number. We regret the error.
4. Conflict of interest: One of the authors was working with the funding agency at the time of the study. This fact was clearly reflected in the manuscript, which provides the institutional affiliation of all the authors and also funding source of the study. We do accept that it would have been more appropriate on our part to declare the above fact in conflict of interest section. Moreover, the said author had moved on to from his job with the funding agency at the time of manuscript preparation.

References

1.	Kapil U, Singh P. Validity of results obtained from thirty clusters on "Prevalence of Iodine Deficiency Disorders" drawn from a large state in india. Indian J Public Health 2011;55:53-4.   [PUBMED]
2.	Pandav CS, Krishnamurthy P, Sankar R, Yadav K, Palanivel C, Karmarkar MG. A review of tracking progress towards elimination of Iodine deficiency disorders in Tamil Nadu, India. Indian J Public Health 2010;54:120-5.   [PUBMED]
3.	Abramson JH, Abramson ZH. Survey methods in community medicine. 5th ed. Edinburgh, United Kingdom: Churchill Livingstone; 1999.    [PUBMED]
4.	Lemeshow S, Robinson D. Surveys to measure programme coverage and impact. A review of the methodology used by expanded programme on immunization. World Health Stat Q 1985;38:65-75.   [PUBMED]
5.	Bennett S, Woods T, Liyanage WM, Smith DL. A simplified general method for cluster-sample surveys of health in developing countries. World Health Stat Q 1991;44:98-106.
6.	Hoshaw-Woodard S. Description and comparison of the methods of cluster sampling and lot quality assurance sampling to assess immunization coverage. Geneva: WHO; 2001. Available from: http://www.who.int/vaccines-documents/DocsPDF01/www592.pdf. [last accessed on 2011 May 19].
7.	International Institute for Population Sciences (IIPS) and Macro International. 2007. National Family Health Survey (NFHS-3), Vol 1. 2005-06, Mumbai, India: IIPS.
8.	Sullivan KM, May S, Maberly G. Urinary iodine assessment: A manual on survey and laboratory methods. 2nd ed. UNICEF, PAMM, 2000.