How many factors should a well-designed experiment test at one time?

1
2
3
depends on the number of experiment variables

In a well-designed experiment, there will only be one factor tested at any one time. If there are more factors than this being tested, the results can become quite convoluted, less focused and more difficult to understand. Although it might seem that the more you test the more you know, this is not the best way to obtain accurate results.