It’s hard to tell how quickly a new technology will expand its reach, but a number of groups in the past year have attempted to predict the future of the rapidly-growing telehealth market.

A report in early 2014 projected the telehealth –– or telemedicine –– market would reach $4.5 billion by 2018. Yet another assessment places the current value of the market at more than $17 billion.

The wide variation in assessments is likely tied to methodology as well as how groups are defining the market. While some groups use the term “telehealth” to refer specifically to devices that patients can use to interact with or receive services from doctors or other health care providers, others include widespread wellness tools, such as wearable fitness trackers.