reflecting the availability of substitute options. The own price elasticity at one level of aggregation can reflect both the own price and cross price elasticities at other levels of aggregation. The interaction between these effects adds significant complexity to the analysis, requiring clarity on which own price and cross price elasticities were measured and controlled for. For example, an analysis of route-level elasticities which does not control for route substitution effects may be more appropriate for a national-level elasticity.