To The Editor,
In a recently published article by Chandra et al. the authors have
stated that they conducted a "case-controlled" study to examine the
anticoagulant effectiveness of warfarin in vitreoretinal surgery.(1)
However, this study was conducted on a cohort of patients receiving pars
plana vitrectomy (PPV); 60 patients who received warfarin (exposure) on
the day of PPV were selected. These 'exposed' patients were matched to
other patients receiving PPV but not taking warfarin (unexposed) on age,
sex, and presenting complaint. These patients were then followed for
perioperative and long term complications. Thus, this is not a case-
control study rather it is a matched cohort study. As a result, the
statistical analysis raises concerns. Matching in a cohort study does not
necessarily eliminate the need for control of matching factors in the
analysis; censoring (competing risks, death, loss to follow-up) can
produce imbalances thereby limiting the extent of matching only to those
among the original counts of persons for whom matching was applied.(2) In
other words, an association of exposure and the matching factors could be
observed for the remaining persons and the observed person-time.2
Furthermore, even if no censoring occurs and one examines risk, control of
matching factors would still be necessary to obtain valid standard
deviation estimates.(3,4) In contrast, case-control studies require
control of matching factors associated with exposure rather than risk.
Moreover, with variable follow-up duration and censoring a matched
survival (time-to-event) analysis producing hazard ratios would have been
a better strategy than reporting the p-values alone; censoring has not
been mentioned in the article.
REFERENCES
1. Chandra A, Jazayeri F, Williamson T. Warfarin in vitreoretinal
surgery: a case controlled series. Br J Ophthal 2010.
http://bjo.bmj.com/content/early/2010/11/11/bjo.2010.187526.full.html.
2010.
2. Rothman K, Greenland S. Modern Epidemiology. 2nd ed. Philadelphia:
Lippincott Williams & Wilkins; 1998.
3. Greenland S, Robins J. Estimation of a common effect parameter
from sparse follow-up data. Biometrics. 1985;41:55-68.
4. Weinberg C. On pooling across strata when frequency matching has
been followed in a cohort study. Biometrics. 1985;41:103-116.
Conflict of Interest:
None declared
To The Editor,
In a recently published article by Chandra et al. the authors have stated that they conducted a "case-controlled" study to examine the anticoagulant effectiveness of warfarin in vitreoretinal surgery.(1) However, this study was conducted on a cohort of patients receiving pars plana vitrectomy (PPV); 60 patients who received warfarin (exposure) on the day of PPV were selected. These 'exposed' patients were matched to other patients receiving PPV but not taking warfarin (unexposed) on age, sex, and presenting complaint. These patients were then followed for perioperative and long term complications. Thus, this is not a case- control study rather it is a matched cohort study. As a result, the statistical analysis raises concerns. Matching in a cohort study does not necessarily eliminate the need for control of matching factors in the analysis; censoring (competing risks, death, loss to follow-up) can produce imbalances thereby limiting the extent of matching only to those among the original counts of persons for whom matching was applied.(2) In other words, an association of exposure and the matching factors could be observed for the remaining persons and the observed person-time.2 Furthermore, even if no censoring occurs and one examines risk, control of matching factors would still be necessary to obtain valid standard deviation estimates.(3,4) In contrast, case-control studies require control of matching factors associated with exposure rather than risk. Moreover, with variable follow-up duration and censoring a matched survival (time-to-event) analysis producing hazard ratios would have been a better strategy than reporting the p-values alone; censoring has not been mentioned in the article.
REFERENCES
1. Chandra A, Jazayeri F, Williamson T. Warfarin in vitreoretinal surgery: a case controlled series. Br J Ophthal 2010. http://bjo.bmj.com/content/early/2010/11/11/bjo.2010.187526.full.html. 2010.
2. Rothman K, Greenland S. Modern Epidemiology. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 1998.
3. Greenland S, Robins J. Estimation of a common effect parameter from sparse follow-up data. Biometrics. 1985;41:55-68.
4. Weinberg C. On pooling across strata when frequency matching has been followed in a cohort study. Biometrics. 1985;41:103-116.
Conflict of Interest:
None declared