Final answer:
Nordstrom's method of using WiFi signals from phones to track customer behavior is a form of market research known as naturalistic observation, which is nonreactive and unobtrusive. This method does not alter the behavior of subjects and relies on convenience sampling.
Step-by-step explanation:
Using WiFi signals from a phone to track customers and give recommendations, as implemented by Nordstrom, is a form of market research. This method employs pre-purchase observational data to refine marketing strategies towards achieving more personalized, one-on-one consumer engagement.
The closest option that relates to this method would be naturalistic observation, as it involves collecting data in a non-intrusive manner without the subject's direct participation, thus it is unobtrusive research. This approach to data gathering is advantageous because it does not alter the behavior of the customers being studied, a concern in direct research methods. Furthermore, it is a way of using convenience sampling to gain insights into consumer behavior, which is valuable for tailoring marketing efforts more effectively.