Logic model
Logic models are hypothesized descriptions of the chain of causes and effects leading to an outcome of interest (e.g. prevalence of cardiovascular diseases, annual traffic collision, etc). While they can be in a narrative form, logic model usually take form in a graphical depiction of the "if-then" (causal) relationships between the various elements leading to the outcome. However, the logic model is more than the graphical depiction: it is also the theories, scientific evidences, assumptions and beliefs that support it and the various processes behind it. Logic models are used by planners, funders, managers and evaluators of  programs and interventions to plan, communicate, implement and evaluate them. They are being employed as well by health scientific community to organize and conduct literature reviews such as systematic reviews. Domains of application are various, e.g. waste management, poultry inspection, business education, heart disease and stroke prevention. Since they are used in various contexts and for different purposes, their typical components and levels of complexity varies in literature (compare for example the W.K. Kellogg Foundation presentation of logic model, mainly aimed for evaluation, and the numerous types of logic models in the intervention mapping framework). In addition, depending on the  purpose of the logic model, elements depicted and the relationships between them is more or less detailed. History of logic models. Citing Funnell and Rogers's account (2011), Joy A. Frechtling's (2015) encyclopedia article traces logic model underpinnings to the 1950s. Patricia J. Rogers's (2005) encyclopedia article instead traces it back to Edward A. Suchman's (1967) book about evaluative research. Both encyclopedia articles and LeCroy (2018) mention increasing interest, usage and publications about the subject. Uses of the logic model. Program planning. One of the most important uses of the logic model is for program planning. It  is suggested to use the logic model to focus on the intended outcomes of a particular program. The guiding questions change from "what is being done?" to "what needs to be done"? McCawley suggests that by using this new reasoning, a logic model for a program can be built by asking the following questions in sequence: By placing the focus on ultimate outcomes or results, planners can "think backward" through the logic model to identify how best to achieve the desired results. Here it helps managers to 'plan with the end in mind', rather than just consider inputs (e.g. budgets , employees) or the tasks that must be done. Evaluation. The logic model is often used in government or not-for-profit organizations, where the mission and vision are not aimed at achieving a financial benefit. Traditionally, government programs were described only in terms of their budgets. It is easy to measure the amount of money spent on a program, but this is a poor indicator of outcomes. Likewise it is relatively easy to measure the amount of work done (e.g. number of workers or number of years spent), but the workers may have just been 'spinning their wheels' without getting  very far in terms of ultimate results or outcomes. However, nature of outcomes varies. To measure the progress toward outcomes, some initiatives may require an ad hoc measurement instrument. In addition, in programs such as in education or social programs, outcomes are usually in the long-term and may requires numerous intermediate changes (attitudes, social norm, industry practices, etc.) to advance progressively toward the outcomes. By making clear the intended outcomes and the causal pathways leading to them, a program logic model provides the basis upon which planners and evaluators can develop a measurement plan and adequate instruments. Instead of  only looking at the outcome progress, planners can open the "black box" and examine if the intermediate outcomes progress as planned. In addition, the pathways of numerous outcomes are still largely misunderstood due their complexity, their unpredictability and lack of scientific / practical evidences. Therefore, with proper research design, one may not only assess the progress of intermediate outcomes, but evaluate as well if the program theory of change is accurate, i.e. is successful change of an intermediate outcomes provokes the hypothesized subsequent effects in the causal pathway. Finally, outcomes may easily be achieved through processes independent of the program and  an evaluation of those outcomes would suggest program success when in fact external outputs were responsible for the outcomes. Various types of logic models. The Inputs → Activities → Outputs → Outcomes template. Many authors and guides use the following template when speaking about logic model: Many refinements and variations have been added to the basic template. For example, many versions of logic models set out a series of outcomes/impacts, explaining in more detail the logic of how an intervention contributes to intended or observed results. Others often distinguish short-term, medium-term and long-term results, and between  direct and indirect results. Intervention mapping logic models. The intervention mapping approach of Bartholomew et al. makes an extensive use of the logic model through the whole life-cycle of a health promotion program. Since this method can start from as far as a vague desired outcome (author's example is a city whose actors decide to address "health issues" of the city), planners go through various steps in order to develop effective interventions and properly evaluate them. There are distinguishable but closely interwoven logic models with different purposes that can be developed through the process: Evaluators thereafter use the  logic model of the intervention to design a proper evaluation plan to assess implementation, impact and efficiency. Advantages. By describing work in this way, managers have an easier way to define the work and measure it. Performance measures can be drawn from any of the steps. One of the key insights of the logic model is the importance of measuring final outcomes or results, because it is quite possible to waste time and money (inputs), "spin the wheels" on work activities, or produce outputs without achieving desired outcomes. It is these outcomes (impacts, long-term results) that are the only  justification for doing the work in the first place. For commercial organizations, outcomes relate to profit. For not-for-profit or governmental organizations, outcomes relate to successful achievement of mission or program goals. Disadvantages. There are some potential disadvantages of logic models due to tendencies toward oversimplification. These include: