In some cases, the ability to collect data has outpaced medical understanding. There is no “normal” testosterone level for an aging male, yet millions of men have been told they suffer from a condition called Low T and are using testosterone gels, even though medical studies have shown the products are dangerous.

In other cases, the data can show up fine, even when the patient is not. You don’t always measure the right thing. When I was in medical school, there was a gallows joke that some patients die with “Harvard numbers.” In other words, the lab tests that were ordered were all perfect, but the patient died anyway.

On the other hand, the test results can look bad even when the patient is fine. Scans of the spine, for example, show that many people have big bulging discs but no back pain. So which do you treat, the M.R.I. or the patient?

One central rule of doctoring is that you only gather data that will affect your treatment. There are now devices that track the activity of your sympathetic nervous system as a measure of stress. But what do you do with that information? Other devices continuously monitor breathing for wheezing that isn’t noticed or audible. Does that matter? Some studies have shown that continuous monitoring isn’t useful for children hospitalized with bronchial infections.

If you were dieting, would stepping on the scale 1,000 times a day help you lose weight? Or consider the treatment of an abnormal heart rhythm. It’s true that constant monitoring for a few days can be highly useful to identify the pattern and what provokes the attacks. After that, though, for many patients a wearable cardiac tracker might simply record normal beats that normal people experience all the time, increasing anxiety for many patients.

Everyone agrees, of course, that useful technology can be lifesaving.

At the Mayo Clinic’s Transform symposium this month in Rochester, Minn., I heard Eric Dishman, a general manager at Intel, explain how he had used data to individualize his own cancer care. More than a decade ago, when he was only partly responding to chemotherapy for a rare kidney cancer, he used a step monitor to help figure out what provoked his pain and then worked with a physical therapist to treat it. More recently, scientists were able to analyze the genetic sequence of his tumor, identifying a medicine for treatment. He is now cancer free.

Likewise, continuous glucose monitors alert sleeping diabetics when their blood sugar drops too low. And some companies are developing chips and other technologies that can perform tests on a drop of blood — perfect for use in remote areas where sterile needles and full laboratories are not available.