Product review (4 stars): Extremely Accurate
I'm a luthier.  Hair hygrometers are very accurate. I ordered this hygrometer, because of the stated +/-3% accuracy.

I got the hygrometer, which had a few very light scratches on the center of the lens.  Rather than send it back to Amazon, I decided to test it's accuracy with a UEI  professional psychrometer.  The hygrometer was off +8%.  I used a very small panhead screwdriver through a hole in the back, to turn a screw, to set the hygrometer to the humidity shown on the psychrometer.  I watched all day as the psychrometer varied between 43-54%, depending on location.  The hygrometer was within 1% of the psychrometer, every time.  Unless the hygrometer quits performing as well, I'm keeping it.  I took off 2 points for the lens scratches and being so far out of calibration.  I added one back for the ease of recalibration and resulting extreme accuracy.  I'll update this is there is a change.