Ordinary Least Square is BLUE with 6 Assumptions should be met. The question: does the simple regression model can fulfil the assumption 3. The answer is depend on the variables weather it is exogenous or indigenous. Exogenous means that the variable influenced by many outside factor that cannot be control by the area of the model, let say the government policy that influence the decision of the company. This case is not violating assumption three that the variance of x is 0. Indigenous is the factor coming from inside the company such as the total assets, revenue etc. The sample linear regression using one of this variables is violating assumption 3. Because 1 variable might not cover several influences.
