The beta coefficient for independent variables surprisingly become negative, and with significant p-value.
e.g. smoking (independent) and risk of lung cancer (dependent) Regression coefficient for smoking is surprisingly negative, where it suppose to be positive (since we expect that an increase in smoking will increase the chance of getting lung cancer)
This is a SIMPLE linear regression analysis, no other variables included.
So, what are the reasons of getting unexpected or wrong sign of coefficient in simple linear regression, considering data cleaning and transformation have been done?