Letters to Editor Control Group and Refusal Rate Critiques about Hepatitis C Virus Infection in HIV Positive Cases Sir, The paper published in this journal about prevalence and risk factors of hepatitis C virus (HCV) infection in human immunodeficiency virus HIV positive cases of a center in Iran(1) has some minor critiques Without a suitable control group, we cannot conclude that even high frequency behaviors like imprisonment (88.9%), injecting drug use (79.2%), or any other variable can be considered as risk factor We should define risk factors in comparison with a basic population Maybe in some populations, such behaviors (variables) are more prevalent and these variables can be considered as protective factor Percentages higher than 50% for example, not mean that this variable is a risk factor Authors did not even compare the total frequency of these demographics and risk factors with other subgroups in their study and mentioned percentages in both HCV positive and negative cases In addition, comparison of HCV positive and negative cases is not completely logical specifically when the number of two groups is imbalanced with 200 HCV-negative cases in comparison with only 26 HCV-positive ones according to ELISA In addition, the nature of sampling shows that the controls are not representative of the HCVnegative cases because of nonprobability sampling and high refusal rate However, the study base of HCVpositive and HCV-negative cases is similar and this similarity rationalizes selecting HCV-negative samples as controls;(2) but, with considering above mentioned issues 251 I know that research in HIV field is most of the time along with considerable missing data due to nonresponse or refusal to participate and even dropout However, high refusal rate may distort the results Authors are encouraged to discuss this issue or at least sensitivity analysis Ali Kabir1,2 Department of Epidemiology, School of Public Health, Shahid Beheshti University of Medical Sciences, 2Center for Educational Research in Medical Sciences, Tehran University of Medical Sciences, Tehran, Iran E-mail: aikabir@yahoo.com References Davarpanah MA, Khademolhosseini F, Rajaeefard A, Tavassoli A, Yazdanfar SK, Rezaianzadeh A Hepatitis C Virus Infection in HIV Positive Attendees of Shiraz Behavioral Diseases Consultation Center in Southern Iran Indian J Community Med 2013;38:86-91 Rothman KJ Greenland S, Lash T Modern Epidemiology 3rd ed Philadelphia: Lippincott Williams & Wilkins; 2012 Access this article online Quick Response Code: Website: www.ijcm.org.in DOI: 10.4103/0970-0218.143032 Indian Journal of Community Medicine/Vol 39/Issue 4/October 2014 Copyright of Indian Journal of Community Medicine is the property of Medknow Publications & Media Pvt Ltd and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission However, users may print, download, or email articles for individual use ...Copyright of Indian Journal of Community Medicine is the property of Medknow Publications & Media Pvt Ltd and its content may not be copied or emailed to multiple... multiple sites or posted to a listserv without the copyright holder's express written permission However, users may print, download, or email articles for individual use