Raymond is completing a study on discrimination in which he has noted that saleswomen who wear skirt suits make more sales than saleswomen who only wear pant suits.​ However, Raymond determined that his findings only pertained to those industries in which the purchasing decision maker is traditionally masculine. For​ example, the sales results were the same for all women selling to medical office​ managers, a female dominated field. Raymond used​ ________ variables to more accurately explain his results.