Program evaluations can play an important role in public policy debates and in oversight of government programs, potentially affecting decisions about program design, operation, and funding. Many different techniques of program evaluation can be used and presented with an intention to inform and influence policy makers. One technique that has received significant recent attention in the federal government is the randomized controlled trial (RCT). This report discusses what RCTs are and identifies a number of issues regarding RCTs that might arise when Congress considers making program evaluation policy. For example, in the 109 th Congress, Section 3 of S. 1934 (as introduced) would establish a priority for RCTs when evaluating offender reentry demonstration projects; Section 114 of S. 667 (Senate Finance Committee-reported bill) would require RCTs for demonstration projects for low-income families; and Section 5 of S. 1129 (as introduced) would call for RCTs for projects and policies of multilateral development banks. Issues regarding RCTs could also arise when actors in the policy process present specific program evaluations to Congress (e.g., in the President's budget proposals) to influence Congress's views and decision making. For many reasons, evaluations often merit scrutiny and care in interpretation. Before discussing RCTs in detail, the report places them in context by discussing (1) questions that program evaluations are typically intended to address, (2) how RCTs relate to other program evaluation methods, and (3) two major roles that Congress often takes with regard to program evaluation. The report next describes the basic attributes of an RCT, major ways to judge an RCT's quality, and diverse views about the practical capabilities and limitations of RCTs as a form of program evaluation. In light of concerns about the reliability of individual studies to support decision making, the report also discusses how RCTs can fit into systematic reviews of many evaluations. The report next highlights two areas where RCTs have garnered recent attention—in education policy and the President's annual budget proposal to Congress. Finally, the report identifies potential issues for Congress that could apply to the highlighted cases, oversight of other policy areas, and pending legislation. Because the vocabulary of program evaluation can be confusing, an appendix provides a glossary with definitions of selected terms. Citizens, elected officials, civil servants, interest groups, and many other participants in governance of the United States have an interest in the performance and results of government programs and policies. To that end, stakeholders might want answers to many questions about programs and policies. For example, how should public policy problem(s) be defined? Is a program addressing some or all of the problem(s)? How well are federal programs and policies managed? What are they achieving? How can they improve? How are stakeholders affected? What unintended consequences might result? In the future, what activities and policies should the federal government pursue in order to best serve the public? What resources should be devoted to a program or policy? In addition, stakeholders might want answers to questions about the quality of evaluations that are brought to policy discussions, given that participants in the policy process will not always advertise weaknesses in studies that also happen to support their policy positions. What might those weaknesses be? Stakeholders might also ask how well federal agencies evaluate the programs they lead and administer. For example, what methods are appropriate to assess a given type of program or policy? Given the available quantity and quality of research, what degree of confidence should be placed in findings, to date? Do agencies have sufficient capacity to evaluate their programs? Are they performing the necessary types of evaluation? Do agencies have sufficient independence to credibly evaluate their programs and policies? What role should agencies play in evaluating programs? At times, many or all of these questions might be of interest to Congress and program stakeholders. All of them will typically be of interest to agency program managers and leaders. Therefore, any of these questions might be potential subjects of congressional oversight or law making. In response to questions like those posed above, program evaluations can be introduced into policy discussions by actors in the policy-making process. These actors—who include organizations and individuals both inside and outside of government—might be interest groups, think tanks, academics, legislators, state or local governments, the President, federal agencies, or nonpartisan institutions. Many actors bring evaluations to policy discussions on their own initiative, oftentimes to emphasize the results or findings that they interpret to support their positions. Some actors (e.g., federal agencies) might bring evaluations in response to legislative or executive branch requirements. Depending on many circumstances, the evaluations that agencies bring might, or might not, support the policy views of the agency's head or the President. When actors bring program evaluations into policy discussions, the studies will oftentimes use different approaches, because there are many possible ways to help answer the questions cited previously. The term program evaluation , therefore, has in practice been interpreted in several ways. For example, there is no consensus definition for the term program . In practice, the term has been used to refer to a government policy, activity, project, initiative, law, tax provision, function, or set thereof. Accordingly, this report uses the term program to refer to any of these things, as appropriate, that someone might wish to evaluate. The term evaluation can seem similarly ambiguous. A recent reference work in the program evaluation literature defined evaluation as "an applied inquiry process for collecting and synthesizing evidence that culminates in conclusions about the state of affairs, value, merit, worth, significance, or quality of a program, product, person, policy, proposal, or plan." Perhaps with many of these considerations in mind, Congress defined program evaluation, for purposes of the Government Performance and Results Act of 1993 (GPRA), as "an assessment, through objective measurement and systematic analysis, of the manner and extent to which Federal programs achieve intended objectives." GPRA requires most executive branch agencies to develop five-year strategic plans, annual performance plans (including goals and performance indicators, among other things), and annual program performance reports. When reporting GPRA to the Senate, the Senate Committee on Governmental Affairs contemplated that not all forms of evaluation and measurement would necessarily be quantifiable, because of the diversity of federal government activities. In sum, program evaluation has been considered in practice, in the scholarly literature, and under GPRA as concerned with investigating both a program's operations and its results. Furthermore, program evaluation has been seen as (1) informing conclusions at particular points in time, and also (2) a cumulative process over time of forming conclusions, as more evaluation information is collected and interpreted. Program evaluations might help inform policy makers, including Members and committees of Congress in their authorizations, appropriations, and oversight work. However, viewpoints about program evaluations can be contentious, both in policy debates and among expert evaluators. In their interactions with Congress, many actors cite program evaluations as part of the rationale for policy changes. In addition, observers and practitioners sometimes disagree about the practical capabilities and limitations of various program evaluation methods, the quality of an individual evaluation, or how a study's findings should be interpreted and used. Therefore, many observers believe it is important for policy makers, including Members and committees of Congress, to be informed consumers of evaluation information when weighing these considerations and making policy decisions. Practitioners and theorists categorize different types of program evaluation (sometimes referred to as different designs or methods) in several ways. Unfortunately, these categorizations are not always consistent with each other, and practitioners and theorists do not always use consistent terminology to describe program evaluations. They sometimes use different definitions for the same term, or use different terms as synonyms for the same definition. This report discusses some of these methods, but does not attempt to provide an overall taxonomy for program evaluation types. The sections below provide basic descriptions of RCTs and other methods. A more detailed description and discussion of RCTs is located later in the report. One program evaluation method that has been a subject of recent interest at the federal level, as well as some controversy, is the RCT. As discussed later in this report, an RCT attempts to estimate a program's impact on an outcome of interest . An outcome of interest is something, oftentimes a public policy goal, that one or more stakeholders care about (e.g., unemployment rate, which many actors might like to be lower). An impact is an estimated measurement of how an intervention affected the outcome of interest, compared to what would have happened without the intervention. A simple RCT randomly assigns some subjects to one or more treatment groups (also sometimes called experimental or intervention groups ) and others to a control group . The treatment group participates in the program being evaluated and the control group does not. After the treatment group experiences the intervention, an RCT compares what happens to the two groups by measuring the difference between the two groups on the outcome of interest. This difference is considered an estimate of the program's impact. The terms randomized field trial (RFT) , random assignment design , experimental design , random experiment , and social experiment are sometimes used as synonyms for RCT, and vice versa. However, use of the word field in this context is often intended to imply that an evaluation is being conducted in a more naturalistic setting instead of a laboratory or other artificial environment. Many other types of program evaluation that are not RCTs can also be conducted in order to address one or more of the questions posed at the beginning of this report. Some of these "other" methods have been called, as a group, observational designs. The term observational design has been used in different ways, but often refers to empirical and qualitative studies of many types that are intended to help explain cause-and-effect relationships, but that do not attempt to approximate an experimental design. Quasi-experimental designs refer to studies that attempt to estimate a treatment's impact on a group of subjects, but, in contrast with RCTs, do not have random assignment to treatment and control groups.