Program evaluation is important to Federal Departments and Agencies as discretionary budgets diminish and departments and agencies are required to justify their programs and budgets with evidence. As shown in the figure below, impact evaluations are distinct among other forms of program evaluation.
The Government Accountability Office defines impact evaluation as:
A form of outcome evaluation that assesses the net effect of a program by comparing program outcomes with an estimate of what would have happened in the absence of the program. This form of evaluation is employed when external factors are known to influence the program’s outcomes, in order to isolate the program’s contribution to achievement of its objectives. 
Because impact evaluations can provide empirical evidence of the causal effects of programs or policies on important outcomes, Federal Agencies often prefer impact analyses to other forms of evaluation when evaluating mature programs. In the following video, Dr. George Cave, Summit's Senior Research Fellow, walks through a generic, but comprehensive, net impact analysis flow chart.