A beautician wanted to know if using 'color safe' shampoo increased the length of time between her clients needing to return to dye their hair again. To determine this she asked a random sample of 45 of her clients whether the shampoo they use is listed as 'color safe' or not. Then, she looked back through her records to determine the length of time in days between their visits to dye their hair. She conducted her study over a 1 month period. Which is a variable of interest in this study?