Management of a retail chain has been tracking the growth of sales, regressing the company’s sales versus the number of outlets. Their data are weekly, spanning the last 65 weeks, since the chain...

Management of a retail chain has been tracking the growth of sales, regressing the company’s sales versus the number of outlets. Their data are weekly, spanning the last 65 weeks, since the chain opened its first outlets. What lurking variable might introduce dependence into the errors of the SRM?