I'm always reminded of this when Theranos comes up. Years ago I was in the hospital, they wouldn't let me out until a blood test showed some numbers that made

I'm always reminded of this when Theranos comes up. Years ago I was in the hospital, they wouldn't let me out until a blood test showed some numbers that made them feel better. I asked about how the one test worked. The nurse would come in and draw a tube of blood (I forget the volume, but if you've had your blood drawn for a test, a typical tube sized - 10's of mL). And then that'd go down to the lab and be mixed with reagents and a tech would look in a microscope slide and count things. Then they'd reverse the math and turn the things they counted into a concentration of stuff in my blood. That's the number the doctor looked at. So when I worked through the process, and deduced that what the lab tech counted was on average 4 particles of interest. And since I was an obnoxious grad student, I brought up shot noise and said that means there's a 50% error in the measurement and that the doctor should discharge me. Of course, he said that even with a 50% buffer, the numbers were too low for him to feel comfortable. Long story, but they needed an entire tube of my blood to count 4 particles in the microscope. A drop of blood would not have enough things in it to count. Real life is discrete, and often we need a big sample just to get enough things to count. That's why I always though Theranos was a fraud.