Anna and Troy’s Weblog

Amy’s comment on my last post sparked my curiosity regarding accuracy vs. precision….one of the many many many things that I once knew in college and have since forgotten……there are an awful lot of those things. I actually think that I have forgotten more things than I remember. I can’t decide if that is a sad commentary on myself, on the educational system, or just a cosmic truth that should be accepted without too much worry…. Food for thought.

For those of you that are interested, see below (it might just be you and me, Amy).

ACCURACY AND PRECISION

ACCURACY

Accuracy describes the nearness of a measurement to the standard or true value, i.e., a highly accurate measuring device will provide measurements very close to the standard, true or known values.Example: in target shooting a high score indicates the nearness to the bull’s eye and is a measure of the shooter’s accuracy. Refer to pictures below:

PRECISION

Precision is the degree to which several measurements provide answers very close to each other. It is an indicator of the scatter in the data.The lesser the scatter, higher the precision. The pictures given below clearly describe Accuracy and Precision.

EXAMPLES

If we measure the length of a foot-ruler and get values of 12.01 in, 12.00 in, 11.99 in, 12.00 in. These numbers are precise enough for us to believe that if we measure it again we would get 12.00+0.01 in. These measurements are precise but necessarily accurate. The foot-ruler may actually be metric ruler of 30.0 cm long. Our measurement is precise but not accurate.

APPROXIMATIONS

Even though physicists usually try for a high degree of precision, there are times when only a close approximation is needed. Physicists some times make rough estimates for making tentative decisions. The accuracy of estimates depends on reference materials available, time devoted, and experience with similar problems.

ORDERS OF MAGNITUDE

Some times physicists make orders of magnitude calculations. In these calculations, the numbers may differ by a factor of ten. Example:106 m 10,980 m differ by two orders of magnitude.