Who performs the test determines the accuracy. If I did it and someone who's needing doing it for 20 years did it his would be way more accurate then mine. But assuming you knkw what your doing they're pretty good.

3-4% margin of error. Just be make sure to be consistent with where you take the measurements and take them in the same place every time. I usually do mine every two weeks or so on Sunday. Wake up, take a piss, weigh myself, measure my waist circumference and take skin fold at at mid axilla(half way between nipple and armpit), 1" to the right of the belly button and mid quad. make sure to take them all on the right side. Usually they come with a book but I use this little calculator

http://www.leehayward.com/free-bodyfat-calculator.htm

if you are measuring correctly in the right spots and your not pressing to hard they are pretty accurate from what I have seen. but where most people make there mistakes are doing it themselves and only measuring in 2 places.. back of tricep.. bicep..on the shoulder blade ..next to the belly button. you need somebody to help you to get an accurate reading