Lurking variable (definition) - a variable having an important effect on the relationship between variables in a study but not included among the variables studied.
Stella, having left her husband and her home, withdraws from the world, constructing a reality outside the norm where she begins to discover her range as a painter. Although others intrude, she is defended and comforted by her companion Matthew, a construct based on a man she met at a gallery opening and whom she does not meet again, in the flesh, until several months later, at the opening of her own work. In the end, the real Matthew Owen will wrest her from her solitude and the comfortable relationship she had developed with her imagined Matthew. Stella’s choice to go away with the real Matthew should leave the audience discomfited. Stella, by severing the relationship with her imagined Matthew, may be abandoning that portion of her psyche that keeps her upright, forward-moving; her source of power and confidence. It might be argued that some people are complete in a circle that includes no one but themselves.