The CBCL/6-18 includes the instruction “For each item that describes your child now or within the past 6 months...”
If you do not change the instruction, you should not administer the CBCL/6-18 more frequently than every 6 months. If you do, the parent may report behavior from overlapping time periods.
The TRF/6-18 instructions specify the previous 2 months. This was done to take account of the fact that teachers may know students for relatively brief periods before needing to assess them. Teachers can be asked to complete a TRF for a student over intervals as short as 2 months.
If you wish to administer the CBCL more frequently than 6 months, you should change the instruction to specify the rating interval that you wish to use, e.g., 2 months. The instruction for the first administration should be changed to the same number of months as the second to avoid sampling different lengths of time for the two assessments.
Avoid asking teachers to fill out forms for many children at one time. If teachers have to fill out forms on too many children, they become less discriminating about which child has which characteristics.
Repeated completion of a form over short intervals can lead to what is called “practice effect,” also known as a “test-retest attenuation effect.” Over brief intervals (e.g. < 1 month), second administration scores tend to be lower than first administration scores. In research on the effects of interventions, it is important to control for such effects by using control groups that receive the same assessment schedules but no interventions or different interventions.