Ok, I want to first want to preface this by saying that I’m a medical doctor, so I do know a thing about pulse oximeters—both in theory and in practice. I noticed the top review has a picture of the pulse ox on a stuffed animal, showing at 99%, as “proof” it is inaccurate equipment.There are a number of problems with this supposition, and it shows a basic lack of understanding of how pulse oxes work. They are equipment that are designed to measure something specific under specific circumstances of being clipped to a fingernail with no opaque polish. NOT inanimate objects like markers and stuffed animals.A pulse ox works by shooting red light across the width of one of the middle fingers. Since tissues are translucent rather than totally opaque, the amount of light that is measured at the receiving sensor measures the proportionality of absorbed red light, which equals the proportion of material that absorbs it. This is called Beer’s Law. Pulse oxes are also governed by another law called Lambert’s Law, which says the amount of light absorbed is proportional to the length of the path that the light has to travel in the absorbing substance. The third law has to do with the fact that oxygenated hemoglobin (which in itself is red) absorbs more light than deoxygenated hemoglobin. This is due to the difference in wavelengths of red and blue light (red objects absorb more red light than blue objects). This is also why the test is not accurate if you have red nail polish on or your finger is not centered and light can move around it (or if you use a tiny pinky finger, for example). You can now see why it’s totally irrelevant how using a pulse ox on an inanimate object (especially, a bright red felt tail!) with totally different properties than an index finger is not a good way to judge accuracy. That’s like saying that your kitchen scale did not work well because you threw it in a swimming pool to measure the weight of the water. Equipment is only designed to work under particular circumstances and limitations.Anyway, I purchased this pulse ox for my father. He has COPD and a bunch of other risk factors for COVID-19, so I appreciate having a heads up if his oxygen sats dip. This is a good vital sign early marker for covid because it is an unusual disease, in that it causes what doctors have nicknamed “happy hypoxia”. You can google this to find out more, but it basically means that in other pneumonias that devolve rapidly into hypoxic conditions that require ventilation, their symptoms also crash, to the point where they lose consciousness or can barely talk. Covid-19 is more insidious and can creep up on some patients, who might be sitting up in bed (yes, symptomatic, but still conscious and talking), fooling even medical professionals into thinking their cases were not too serious until it was too late. This is the reason medical professionals find pulse oxes to be useful—they measure signs of disease (objective results from testing) rather than symptoms (subjective reports of feelings from patients). It gives me peace of mind to know my father has this and can measure it daily when he does his insulin shots and measures BP. He has chronic bronchitis, diabetes, stroke history, high blood pressure, obesity, and he is nearly 80, so he’s definitely in that very high risk category.On the actual unit, it was extremely easy to set up and use. You press a button and it turns on (once you put the batteries in the right way, lol); turns off automatically. I didn’t find anything inaccurate about it... and the hardest part of it was probably getting the lanyard in, haha (I advise using a needle to get the other end out of the hole; there’s no way a senior citizen could do this alone, so please help them). I’ve used many pulse oxes in my day. The professional ones in the hospitals are about 1% more accurate than these little portable, cheap pieces of equipment that you can buy for the home. I highly recommend them for any patients or elderly who have risk factors or disease comorbidities (especially, pulmonary). These little portable ones were sold out for months during the pandemic first peak (even hospital staff had trouble getting their hands on privately sold ones), so I’m glad to have one now.