Attribute-level balance within blocks

[ashani.abayasekara]

Hi,

I'm designing a labelled discrete choice experiment with six attributes, with levels ranging from 2-4. The design currently has a total of 72 rows, where each respondent will be given 8 choice sets each from 9 blocks. While 12 choice sets per individual would be the best for full attribute-level balance, we feel this might be too much of a cognitive burden.

To maintain attribute-level balance as much as possible, I understand that I can specify the number of times (or a range of times) each level of an attribute should appear for the total number of rows, by indicating these numbers in brackets alongside the relevant attribute and levels in the utility model. My question is whether the same can be done to ensure that each block also maintains level balance to the maximum extent possible? Specifying this for the total number of rows does not guarantee that it holds within each block.

Relatedly, is maintaining attribute-level balance within each block necessary, or is it sufficient if this is only ensured for all choice sets as a whole?

Thank you very much.
Ashani

[Michiel Bliemer]

First of all, neither attribute level balance within a design, not attribute level balance within a block, is a requirement. They are merely nice properties to have, but in many cases it is not possible to achieve a high degree of balance within a block.

I usually aim for a high degree of attribute level balance within a design (although not always perfectly balanced), but I usually settle for a lower degree of level balance within a block. Only with orthogonal designs you can get perfect attribute level balance within a block. With random or effect designs, the ;block properties optimises for attribute level balance within a block, but it will never be perfect, and it may be far from perfect. You cannot specify any additional minimum-maximum appearance of levels within this optimisation process.

If you are not satisfied with the way that Ngene has blocked the design, you can of course always manually try to assign the choice tasks to blocks based on your own definition of measuring attribute level balance within a block (there are many different ways of measuring this). Blocking does not affect the efficiency of the design.

Michiel

[ashani.abayasekara]

Hi Michiel,

Thank you very much for your explanation. It's reassuring to know that attribute level balance is not a strict requirement. I may adjust some of the choice tasks manually within blocks to achieve better balance.

Best,
Ashani

[ashani.abayasekara]

I have another follow-up question on attribute-level balance. I'm trying to understand what the option "imbalance" in the Ngene syntax aims to do. Is that an option to minimise any potential imbalance in a design? For example, the syntax "eff = (mnl,d) + 0.5*(imbalance)".