Response to 'Correspondence on 'Comparative effectiveness of first-line tumour necrosis factor inhibitor versus non-tumour necrosis factor inhibitor biologics and targeted synthetic agents in patients with rheumatoid arthritis: results from a large US registry study' by Zheng

We thank Zheng et al 1 for their interest in our study2 and for providing constructive comments. They mention that even though the baseline mean Clinical Disease Activity Index (CDAI) was not statistically different, the proportion of patients with moderate or high disease at baseline was different and this could have been a key effect modifier.1 We used standardised differences to evaluate variables at baseline. Calculation of the standardised difference not only accounts for the means of the two cohorts, but also the SD. We noticed that continuous CDAI was balanced across the two therapy classes (as listed in the manuscript table 1, SD=−0.015)2 and similarly observed for categorical CDAI (low/moderate/high; SD=0.0515; results not shown …