Errors in Experimental ResultsIf you prefer a tutorial in American units Click Here.
Whenever one measures anything, there is an error associated with the measurement. If you try to measure the distance between two points with a ruler, you might find that it is 5 cm. On your ruler, centimeters are subdivided down to milimeter markings. If you look carefully, you might see that the distance is actually between 4.95 and 5.05 cm (half a tic-mark up or down).
The range of values around the measured value is the error in the measurement - in this case, from 4.95 to 5.05 cm. The result is written as 5.00 ± 0.05, implying that the correct result is anywhere in the range of 5 - 0.05 ( = 4.95) to 5 + 0.05 ( = 5.05). ±0.05 is "the error" in the measurement.