Sentence:  In America alone, people spent $170 billion on “direct marketing”—junk mail of both the physical and electronic varieties—last year. Yet of those who received unsolicited adverts through the post, only 3% bought anything as a result. If the bumf arrived electronically, the take-up <WSD>rate</WSD> was 0.1%. And for online adverts the “conversion” into sales was a minuscule 0.01%. 
Meaning of the word marked with <WSD> tags:  The proportional relationship between one amount, value etc. and another. (from the 15th century)