We will conduct a CBC study with two attributes, model and price.
Model has 27 (yes 27!) levels, and price has 7.
We already advised the client that this is too much, but it is what it is.
The last project we did for the same client has 25 levels for model and 5 for price, we did it with 18 tasks showing 7 concepts each.
Now Im testing the design for this one, and with the same number of tasks the "strenght" of the design is sbout 60% of that of the previous project.
I went and increased the number of tasks to 22 and got this message:
"The strength of this design could not be calculated (the covariance matrix has a determinant of zero)"
I also tried with 27 and 20 tasks, all provide me the same message.
1. What does this message mean?
2. By any chance is this an indication that we definitely should not go over 18 tasks?