It started in the '70s. A pregnant woman with diabetes faced a deeply entrenched medical routine: months in hospital having her blood glucose closely monitored. She managed to stay out by convincing her doctor, Clara Lowy,  to let her monitor her own glucose levels. Around that time others had been starting down the road of combining wearable sensors  with wearable computing.
Since then, medical self-monitoring has galloped along. And somewhere along the line, using sensors feeding data into computers became a normal part of life for high-performance athletes. Now, many of our friends are clocking their every step. There are people using apps to monitor their sleep, food, weight, heart-rate and moods - and even some people without diabetes are buying blood glucose monitors.
If the enthusiasts of the 'quantified self' movement  are right, this mash-up of health, fitness, gadget and data-crunching crazes is just the beginning of a very big wave that will carry most of us along.  And they rather fervently believe that the more people juggling more data, the better.  We'll prevent illness, identify the causes of any ills or irritations we do have by constant cross-analysis of our very own big data, and generally hit any number of health and fitness targets. What could possibly go wrong?
Professor Paul Glasziou, author of Evidence-Based Medical Monitoring,  is taking stock this week at Evidence Live  in Oxford. Medicine has come a long way since Clara Lowy's first foray with a patient monitoring her own blood glucose. Glasziou pointed out that billions are now spent annually on blood glucose monitoring. And while it's valuable for people with diabetes that are pregnant and/or using insulin, it isn't going to improve the health of those who don't use insulin. 
Glasziou stressed the harm caused from acting on random within-person variations. For monitoring to be effective, he said you need 3 elements:
- Valid and accurate measurement,
- Informed interpretation and
- Effective action that can be taken in response.
And you need evidence to show that a monitoring regime does more good than harm.
How valid and accurate are the tracking devices for healthy people? It's clear that even the basic ones still have a long way to go on this score.  Using two trackers on the same day yielded a count of 7,116 steps from one device and 2,192 steps for one enthusiast who conceded this isn't yet going the way you'd like. 
Whichever pedometer is used though, there's an underlying problem in the assumption that they will improve health. Pedometers may help you walk more when you start using them, but improvement hasn't been solidly sustained over time.  The same problem may hamper the hopes people have for benefiting from monitoring their weight. 
Glasziou's informed interpretation requirement is another big problem with relying on analyses from big data. There are major pitfalls in acting on random variations and the results of multiple testing.  With large datasets, a great deal that may not be important could appear to be "significant". 
What's more, we are still by no means as well-informed about truly helpful targets for the healthy as many appear to think.  Human health isn't about simple mechanics and tinkering with a few measurable levels.
All this skepticism doesn't mean that I believe this is all a waste of time. Far from it. There will no doubt be ways that monitoring and monitoring technologies will help many people. There is, though, potential for harm, including unnecessary and pointless anxiety.
There's value, too, in contemplating the meaning of where we're going with this, and the consequences of adults focusing so much on our selves in this particular way.
When Clara Lowy's patient turned to self-monitoring, it was to gain some freedom from medicalization. Self-monitoring still provides that freedom and indeed empowerment for many people with chronic diseases.
But as it encroaches on the healthy, monitoring becomes a beachhead in the medicalization of our lives, and maybe even how we view our selves and what constitutes a life well lived. Socrates spoke of the examined life being the only kind worth living for a human being:  he wasn't expressing a belief in self-improvement from turning our attention ever more to the minutiae of our bodies.
Footnote: Pew Internet researchers  estimate that 1 in 5 Americans is using some kind of computer technology to "self-track" their health.
Image: copyright by author.