We agree with statistical interpretation of the results
made by Dr. Dutta. However, conclusions and interpretation made in our
study must be viewed in light of some important facts. In absence of
concrete data on duration of phototherapy in Rh hemolytic disease,
calculation of sample size was based on our pilot data (unpublished). We
enrolled the pre-calculated number of subjects, but because of wider
dispersion of phototherapy duration, the primary outcome of our study, we
were unable to reject the null hypothesis that 1 g/kg of IVIg is not
better than 0.5 g/kg in reducing duration of phototherapy. Post-hoc
analysis showed that the study was underpowered to detect a difference of
24 h in the duration of phototherapy- the intended difference. But the
study had 80% power to detect a difference of 36 hr in the duration of
phototherapy. The trend towards decreased duration of phototherapy
observed in 1 g/kg IVIg group should be interpreted with caution in light
of the opposite trend of longer hospital stay in the same group and the
illness severity of babies. Moreover, we were unable to detect significant
difference in other outcomes like number of exchange transfusions,
duration of hospital stay, number of packed red blood cell transfusions
and peak serum total bilirubin. Our study was not powered to detect change
in these outcomes, but detecting statistically meaningful difference in an
important outcome like need of exchange transfusion will need huge sample
size.
Furthermore, even a small difference will achieve
statistical significance if sample size is big enough. Statistical
significance testing does not reflect the magnitude of the effect, and the
term "statistically significant difference" does not denote that the
difference between a test and control group was clinically meaningful with
regard to a desired outcome. We agree that reporting of no dose-effect
relationship between IVIg and duration of phototherapy may not be correct
statistically, but we based our conclusions on utility of clinical benefit
than what our study was powered enough to detect.