We bought the Grobag Egg and have found that it's about 2 degrees C higher than it should be. I'm sending it back to the manufacturer. I emailed them and they replied saying that it should be accurate from between + or - 1 degree C.
Apparently they test all the Eggs that come back to them in a `draft proofed box for approximately 3 hours with a thermocouple attached to a calibrated Digitron thermometer' and observe the temperature readings through a window.
They apportion typical reasons for inaccurate readings down to customers handling the sensor, errors in other measuring devices, differences in air current and sunlight!
All I can say is that ours sat undisturbed out of direct sunlight on a bedside table, in a small bedroom, that had a window and door - much like most peoples nursery.
I tested the Egg against a mercury thermometer, a digital one and our room Digistat. The Egg was 2 degrees C over the other 3.
I think perhaps the manufacturer should test the Egg in a real life environment and not a lab, and should not set the display to read to 1 decimal place when they only regard it accurate to + or - 1 degree C.
It's not something that gives us peace of mind and in my view it's a fun gadget only and should not be relied on for accurate room temperature readings. We paid £20 for it and that's too expensive for what is really only coloured light. Even if they got the accuracy to within + or - 1 degree C, that's a significant temperature difference and you can get more accurate thermometers for a lot less.
I await the results of their test on our Egg.