Lesson 2

Precision

Concept of Precision

Accuracy and precision are concepts that you are already familiar with. In day to day living their importance varies considerably with the situation. If you are describing a window in terms of how it relates to its room, then you might say it is four feet by three feet in size. Not very precise but it gets the message across. On the other hand, if you were giving someone the dimensions to cut a piece of glass so that you could replace a broken window, then saying 4 ft x 3 ft would not be nearly precise enough. If it is cut 1/16th of an inch too large, it won't fit in the frame. If it is cut too small, it won't fill the frame and will either fall through or not seal properly.

Note that whether we deal with general dimensions or replacing the glass, the precision necessary for an adequate description was dictated by the situation.

Also note that in each case, the numbers used to describe the size were rounded off. Any measured value (except for counting) cannot be described exactly. Measured values are rounded off in a way that depends on how they are measured. Where the number is rounded off is its precision. Whether it is measured correctly is its accuracy.

Since chemistry is a new situation for you let me explain what is expected of you when you make and record measurements. Write down measured values to the same degree of precision with which you measure them.