In addition, observational studies (e.g., in biological or social systems) often involve variables that are difficult to quantify or control.
Dǎɣ tiwat, teɣareten ən akayad (as milhaw, dǎɣ imartiyan ən tamudre meɣ timətawen) tagashan agud iyan imətuyan assohatnen aɗinen meɣ ategaznen.