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How to randomize items and blocks
This wiki post will cover a few types of common randomization schemes:
- Simple randomization of items
- Randomizing blocks of ordered items
- Randomizing items within ordered blocks
- Randomized items within randomized blocks
All of these schemes can be implemented using the item_order and block_ordercolumns within your survey spreadsheet.
The item order column has a very clear interpretation: it's the order the items will be displayed (1st, 2nd, 3rd, etc). Items assigned the same item_order value will be randomized.
The block_order column is a bit less obvious. It is really more like a "block assignment" variable. Entries in this column should be 1-4 letters ("A", "BB", "PRE", "POST", etc). Although we often think of letters as ordered, the block_order variable doesn't interpret them this way. Instead item (and block) orders are determined by the item_order variable.
The interaction of the variables is best demonstrated through examples.
This will display an instructions trial (instr) followed by items 1-4 in a randomized order.
| name | block_order | item_order |
|---|---|---|
| instr | 1 | |
| item_1 | 2 | |
| item_2 | 2 | |
| item_3 | 2 | |
| item_4 | 2 |
Here, items 1-4 will be displayed in order within two randomly presented blocks.
| name | block_order | item_order |
|---|---|---|
| item_1 | A | 1 |
| item_2 | A | 2 |
| item_3 | B | 1 |
| item_4 | B | 2 |
The following items will be displayed across two pages. Block A will be displayed first, with item_1 and item_2 displayed in random order and a submit button displayed below them (instructions could also be placed at the beginning of the page in a similar fashion).
| name | block_order | item_order |
|---|---|---|
| item_1 | A | 1 |
| item_2 | A | 1 |
| submit1 | A | 2 |
| item_3 | B | 3 |
| item_4 | B | 3 |
| submit2 | B | 4 |
Below is a fully randomized design with blocks of items each on their own pages.
Items 1-4 will be displayed in random order within blocks. The blocks will also be randomized, with each block appearing on its own page. A submit button will be displayed on the bottom of each page (a similar technique could also be used to place instructions at the beginning of the page in a similar fashion).
| name | block_order | item_order |
|---|---|---|
| item_1 | A | 1 |
| item_2 | A | 1 |
| submit1 | A | 2 |
| item_3 | B | 1 |
| item_4 | B | 1 |
| submit2 | B | 2 |