图中的设计为包含三个备选项（一个什么也不选项）的设计，现在需要做一个只包含两个备选项（一个属性选择项，一个什么也不选项），当我把nalt(2)改为nalt(1)时，系统显示“The random starting design does not identify all of the effects to be estimated.
Possible options include:
- Simplify the model
- Increase the number of choice sets in the design
- Run dcreate again specifying a different random number seed
” 求大神解答疑惑