Flatirons Surveying has a great page that describes the difference between accuracy and precision.
Precision: the degree of refinement in the performance of an operation, or the degree of perfection in the instruments and methods used to obtain a result. An indication of the uniformity or reproducibility of a result. Precision relates to the quality of an operation by which a result is obtained, and is distinguished from accuracy, which relates to the quality of the result. ...Accuracy: the degree of conformity with a standard (the "truth"). Accuracy relates to the quality of a result, and is distinguished from precision, which relates to the quality of the operation by which the result is obtained.
So precision tells you how good your method is, and accuracy tells you how good your measurement is. As Yiding Wang is quoted at the top of the page, "Accuracy is telling the truth . . . Precision is telling the same story over and over again."
There are also diferent statistical methods to measure precision (standard deviation/relative standard deviation)and accuracy (bias and relative bias).
It's easy to think of this with bathroom scales, especially the old-fashioned mechanical kind. Lots of people keep those set to measure several pounds lighter than the true weight. But such a scale is still very useful for monitoring changes in weight over time.
Pirate: Please exlaborate.
BB: I just wish I could find out how precise and accurate mine is....
Well they all initally relate to errors in measurment typically accuracy and percision are edealt with in terms of error.
First off we would have Indeterminate Error (also called Random Error) it accounts for uncertainty in measurments and serves as a measure of precision.
It is represented by Standard Deviation and typically it is calculated:
s = [Sum(Xi-X)^2/N-1]^(1/2)
Where:
Xi = a given observation
X = average of all observations
n = number of observations
Note: N-1 is used for typically fewer than 20 observations, where as more than 20 would just use N
The other is Determinate Error (also called Method Error its an absoulte measure of accuraccy.
It is represented by Bias and is typically calculated as the average of all observations minus the accepted value.
E = X-Xt
X = average of all observations
Xt = actual value
Looking at the equations its not hard mathematically to see the relation to the sample. And remember you can always measure precision, but you can't always measure accuracy, because no matter what left wingers say there are cases where one answer is correct.
You could quite eaily determine how accurate yours is with a known weight and repeated weighings. But you would also want to Q-Test your data to make sure you didn't have any whacky reading that you could statistically justify discarding.