This article addresses some of the challenges faced when attempting to evaluate the long-term impact of informal science learning interventions. To contribute to the methodological development of informal science learning research, we critically examine (Falk and Needham (2011) "Journal of Research in Science Teaching," 48: 1-12.) study of the California Science Center's long-term impact on the Los Angeles population's understanding, attitude and interest in science. This study has been put forward as a good model of long-term impact evaluation for other researchers and informal science learning institutions to emulate. Moreover, the study's claims about the Science Center's positive impacts have been widely cited. This essay highlights the methodological limitations of Falk and Needham's innovation of using an indicator-based impact measure (a "marker") designed to limit their reliance on self-report data, and points to more valid options for assessing long-term learning or attitudinal impacts. We recommend that future research employ more direct measurements of learning outcomes grounded in established social scientific methodology to evaluate informal science learning impacts.