The small letters on the path which represents the strength of causal effects. To be specific, it is the amount of variability that can be accounted for by the source variable.  

A modern interpretation of it is different. It is now considered a result of hypothetical inventions on the source variable.

University of Michigan - Ann Arbor

The path diagram Wright used to analyze inheritance of fur color in guinea pigs. 

D denotes developmental factors, E denotes environmental factors, G means the genetic factors from each individual parent, H means the combined hereditary factors from both parents, and O stands for offspring. The small letters, called path coeffecients, represent the strength of effects.

The path diagram for inheritance of fur color in guinea pigs

A path diagram to show the model involving technology use, student engagement, self-directed learning and academic performance.  Here, path analysis allowed researchers to combine causal theory and multiple regression to assess the direct and indirect effects between variables of interest. 

Path Diagram for technology use on self-directed learning, student engagement and academic performance

Pearl, J., & Mackenzie, D. (2019). The Book of Why. Penguin : 
[search.lib.umich.edu/catalog/record/016899333?query=the+book+of+why&library=All+Libraries](https://search.lib.umich.edu/catalog/record/016899333?query=the+book+of+why&library=All+Libraries) 

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