Castor uses a validated variable block randomization model. This randomization algorithm is constructed in such a way that randomized inclusions are divided across groups (with optional stratification) in variable block sizes. This is done to ensure true randomness during the allocation.
You can define your groups and choose the block sizes in the study settings. The provided selection of block sizes is dependent on the number of groups and their assigned weight. When you begin randomizing, a block size will be determined randomly from this selection for each stratum.
As inclusions are divided among different block sizes, it may initially appear that the division is unequal, but eventually the distribution will be balanced. An exact 1:1 balance is only possible if all created blocks are allocated at the end of data collection. This is unlikely, therefore a slight imbalance should be expected.
We recommend consulting a statistician to ensure you are selecting the right block sizes.
You can also read more information about randomization on the Castor blog.