Attribute level coverage across blocks

[Elisabeth_H]

Dear all,

I am generating an efficient design for a project.

One of the key attributes that we are focusing on has 10 levels. Because this is an important attribute for us, we would like to have coverage (i.e. non-zero appearance of levels) for this attribute at both the aggregate and block level.

Currently among all the different designs generated (10 designs with different random seeds), I have balance across all 10 levels but when you look at the levels from the block level (there are 5 blocks), I have at least level that is zero occurance in a block (i.e. when I crosstabulate the attribute with block, there is at least one cell with 0 frequency).

I know there exists a command to ensure an overall (aggregate) minimum occurance frequency for attribute levels, but I can't find anything in the manual for if we want to also ensure this is ensured at the block level. I was wondering if there is a command that can ensure I have non-zero occurances within a block or something similar?

I appreciate your help.

Best regards,
Elisabeth

[Michiel Bliemer]

The only way to guarantee attribute level balance within each block is by using a (simultaneous) orthogonal design. It is not possible to guarantee attribute level balance within an efficient design because "true blocking" only exists for orthogonal designs, although Ngene will try to have as much attribute level balance as possible within a block.

You can try the following things:

1. Pllay around with the blocking settings to see if that changes anything, see page 229 in the Ngene manual. Try adding the minsum or minmax optional commands.

2. Swap choice tasks manually across blocks to improve attribute level balance.

3. Require that the design is orthogonal, i.e. add ;orth = sim to your syntax (you can keep ;eff as Ngene will try to find a design that is both efficient and orthogonal).

Michiel

[Elisabeth_H]

Thank you Michiel,

I don't actually require balance, just non-zero occurance of levels across blocks. I'll have a look at your suggestions.

Thanks,
Elisabeth

[Michiel Bliemer]

But with an attribute with 10 levels, it will be quite difficult to have non-zero occurance (assuming that you are only have 10-14 choice tasks or so per block to avoid fatigue).

Manually swapping choice tasks is usually the easiest. You can also manually change some attribute levels if needed, small changes should not affect design efficiency much.