LayEv (Layered Evidence) is an interactive visualization system designed to explore the results of systematic evidence synthesis. Each layer corresponds to the reported effects of an individual empirical study. The input data has to be provided as digraph objects via text files with a .dot extension. A digraph object contains a name, typically denoting the authors and publication year of the study, and a body specifying the graph data. Each line specifies a connection between two variables (nodes), connected by a directed edge indicated by an arrow (->). Undirected relationships are specified by including a pair of directed edges covering both directions. Additionally, each edge includes a type that specifies the nature of the relationship: "sp" for a significant positive relationship, "sn" for a significant negative relationship, "s" for a significant relationship where the sign is unknown, "a" for analyzed relationships were no significant effect has been found, and "m" for measured relationships that have not been analyzed. An example of a valid .dot file is shown below.
digraph Example {
"B" -> "D" [type="sp"]
"D" -> "B" [type="sn"]
"C" -> "B" [type="s"]
"A" -> "D" [type="a"]
"A" -> "B" [type="m"]
}