I have daily sales data and stock availability for items in a supermarket chain. My goal is to estimate the sales quantity elasticity with respect to availability (represented as a percentage). With this model, I want to understand how a 1% change in availability impacts sales. Currently, single-variable regressions yield low R-squared values. Should I include lagged sales values in the regression to account for other endogenous factors influencing sales? This would isolate availability as the primary exogenous variable