Product review (5 stars): Your mileage may vary...
I was suspicious of this monitor for RH because it read so much higher than three other devices I own. I have a brand new Frigidaire dehumidifier with an electronic display, an Accurite 00613A1 that I bought recently at the same time when I bought the Vicks (to compare against and because the Accurite had a magnet mount on the back), and a 10-15 year old Bionaire unit. All had temperatures within a point of each other. The Bionaire and Frigidaire seemed to be closest to each other on relative humidity readings.  But the Accurite was 3-4 percentage points higher than the Bionaire and the Vicks was 9-10 points higher than the Bionaire.  I did the salt-based RH test where you put a good quantity of plain dampened salt in a bowl inside a gallon baggie for 8 hours or so with the monitors inside next to the bowl.  I did this with the Vicks and Bionaire only (the other units are at another location now) and the Vicks was dead-on as it is supposed to be at 75%.  The Bionaire was low at 67%.  There may be a difference in accuracy at the different humidity ranges or temperatures but this test led me to believe the Vicks has the most dependable readings.