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- Basic randomization
- Blocked randomization
- Blocked randomization with Stratification
Basic Randomization: (eg: CHOP Weight Loss, Way to Walk)
- This type of randomization procedure divides participants equally across groups regardless of individual characteristics. This process will result in approximately equal group sizes.
- Basic randomization can be accomplished by altering information contained in the settings section for each arm of your particular study.
- Basic randomization should only be used if you have a medium to large “n” and if you expect that all participants will be randomized in a short period of time (less than 2 months).
Blocked Randomization (ex: CHF EMPOWER)
- This procedure ensures close balance of participants in each group at any time during the study enrollment period.
- A “block” is a small group of participants assigned to the arms within a study. The minimum block size is equal to the number of study arms.
- If the randomization is balanced (meaning that the same number of participants is assigned to each arm), the block size must be a multiple of the number of arms. For example, a four-arm study could have block sizes of 4, 8 12, etc. In a block of size 4, 1 participant is assigned to each arm. In a block of size 8, 2 participants are assigned to each arm.
- If the randomization is unbalanced (meaning that some arms will have more participants than other arms – see “Arm weights” below), the block size must be a multiple of the sum of the arm weights. For example, a three-arm study in which two intervention arms each accrue twice as many subjects as the control arm (arm weights of 2, 2, 1) could have blocks sizes of 5, 10, 15, etc. In a block of size 5, 2 participants are assigned to intervention 1, 2 participants are assigned to intervention 2, and 1 participant is assigned to control. In a block of size 10, 4 participants are assigned to intervention 1, 4 participants are assigned to intervention 2, and 2 participants are assigned to control.
- For studies will a small “n”, you are advised to use smaller block sizes (e.g., 2 and 4 for a study with 2 arms and 30 per arm). For larger studies you may use larger block sizes (e.g., 4 and 8 for a study with 4 arms and 200 per arm).
- Blocked randomization should be used in most circumstances and is preferred in order to keep randomization balanced over time. If your anticipated sample size is small or you anticipate a long recruitment period you should use blocked randomization to avoid confounding by temporal trends (e.g., changes in practice patterns over time or a shift in the kinds of patients being referred).
Blocked Randomization with Stratification (ex: CMMI HeartStrong)
- This approach to randomization is the same as blocked randomization, except that prior to randomization, participants are placed into strata (i.e., categories) on the basis of one or more chosen criteria (e.g., gender, BMI, age); they are then randomized to study arms according to set block sizes as described above. Participants will be allocated proportionally to each intervention while controlling for the chosen criteria so that there is an equal number of participants with each stratification factor in each intervention arm. For example, if you want to use gender as a stratification factor, this process will allocate an equal number of women and men to each intervention arm.
- On the randomization screen select "Block Randomization With Stratification" and click the "Update Configuration" button.
- Once this has been done a new link will appear in the Study Configuration navigation titled "Configure Strata"
- Clicking on this link will take you to a page where you can configure the strata groups for your study. To create a stratum click on the "New" button at the top of the page. When creating strata there are two parameters that can be set: "name" and "block sizes." The name parameter is used across the site as a way to differentiate different strata from each other. The block sizes parameter determines what the potential block sizes are for that stratum.
- On the main strata configuration page each stratum for a study is displayed, as well as the total number of participants that have been randomized into each arm in the current randomization block, the totals for all blocks, and the potential block sizes for each stratum.
- Once all the needed strata have been created the feedback tool can be used to configure how participants are placed in strata. Any encounter during enrollment can be used to assign a participant into a stratum.
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- W2H Admins can see the current state of the blocks.
- W2H Coordinators can’t see current state of the blocks so they will remain blinded to how the blocks are being filled
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