Static block randomization builds a balanced, predictable allocation sequence inside Datacapt, using variable block sizes. Each block distributes participants equally across arms, which keeps the groups balanced while leaving the next allocation unpredictable.
It is called static because the full list is generated once, before the study starts, and follows your block rules from then on.
🔑 Prerequisite: before you start, you need the permission to configure randomization, and five decisions already made.
Your treatment arms, for example Treatment A and Treatment B.
Your allocation ratio, for example 1:1 or 2:1.
The randomization method: Static, Dynamic or Import.
The randomization type: Open label or Double blind.
Any stratification factors, for example center, gender or age range.
Turn randomization on for the study
🛠️ Step by step guide
Open your study.
Go to Study settings, open the RTSM tab, and activate Randomization.
Create and configure the randomization
🛠️ Step by step guide
Go to the RTSM tab in the study menu.
Create your randomization and give it a name. That name is what you will see in the eCRF Builder and in trial supply management when configuring allocation, so make it recognizable.
Set up the arms: name each one and define the allocation ratio between them, for example 1:1 or 2:1. Each arm gets a unique ID, generated automatically, which you can use in the eCRF.
Set the method to Block and define your block size. Block sizes are always a multiple of the number of arms, and are proposed automatically.
Set the randomization type, which decides whether the assigned arm is displayed to users.
Define the scope and stratification: per site for independent sequences per site, or per stratum if you use stratification factors.
A 1:1 ratio with a block size of 4 produces 2 allocations per arm in every block. If you stratify, Datacapt generates a separate list per stratum.
Add the randomization action to the eCRF
The configuration alone does nothing visible. Somebody has to be able to press a button, and that button is a question type in the builder.
🛠️ Step by step guide
In the eCRF Builder, open the form where the action should sit.
Click Add question and choose # Randomization.
Rename the button label if you want to.
Save.
Test it before you go live
🛠️ Step by step guide
In your draft study, generate a new test eCRF.
Click the Randomize action button.
Confirm by selecting Randomize.
Repeat with several test subjects, so you can see the allocation logic working across a block rather than once.
💡 Tip: if the configuration is not right, go back to the randomization settings, adjust and save. Be aware that saving a change resets the randomization to zero, so your tests start again from the first slot. Once you are satisfied, stop changing it and finalize the study for live deployment.
What to know once it is live
Blinding is handled by permissions. Users without the right permission do not see the assigned arm.
Already have a list? You can import it into Datacapt instead of generating one.
⚠️ Warning: once static randomization is activated on a live study, the list cannot be modified. Changing it means uploading a new version. Treat the moment you activate as the point of no return, and be sure your testing is finished before you reach it.