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 offered selection of block sizes depends on the number of groups and their weight. When you start randomizing, a block size will be determined randomly from this selection for each stratum.
Because inclusions are divided among different block sizes, when you start randomizing it might seem that the division is unequal, but eventually the distribution will be balanced. An exact 1:1 balance is only possible if all created blocks were used up in the end, which is often not the case, so a slight imbalance should be expected.
Always consult a statistician to ensure you are selecting the right block sizes!
Read more about randomization on our blog.