There is no such thing as a
perfect measurement. Each measurement contains a degree of uncertainty due to the limits
of instruments and the people using them. In laboratory exercises, students are expected
to follow the same procedure that scientists follow when they make measurements. Each
measurement should be reported with some digits that are certain plus one digit with a
value that has been estimated.
For example, if a student is reading the level of water in a
graduated cylinder that has lines to mark each milliliter of water, then he or she should
report the volume of the water to the tenth place (i.e. 18.5 ml.) This would show that the
18 mls are certain and the student estimated the final digit because the water level was
about half way between the 18 and 19 mark.
Two concepts that have to do with measurements are accuracy and
precision.
The accuracy of the
measurement refers to how close the measured value is to the true or accepted value.
For example, if you used a balance to find the mass of a known standard 100.00 g mass, and
you got a reading of 78.55 g, your measurement would not be very accurate. One
important distinction between accuracy and precision is that accuracy can be determined by
only one measurement, while precision can only be determined with multiple measurements.
Precision refers to how
close together a group of measurements actually are to each other. Precision has
nothing to do with the true or accepted value of a measurement, so it is quite possible to
be very precise and totally inaccurate. In many cases, when precision is high and
accuracy is low, the fault can lie with the instrument. If a balance or a
thermometer is not working correctly, they might consistently give inaccurate answers,
resulting in high precision and low accuracy.
A dartboard analogy is often used to
help students understand the difference between accuracy and precision. Imagine a
person throwing darts, trying to hit the bull's-eye. The closer the dart hits to the
bull's-eye, the more accurate his or her tosses are. If the person misses the
dartboard with every throw, but all of their shots land close together, they can still be
very precise.
You must strive for both accuracy and
precision in all of your laboratory activities this year. Make sure that you
understand the workings of each instrument, take each measurement carefully, and recheck
to make sure that you have precision. Without accurate and precise measurement your
calculations, even if done correctly, are quite useless.
Now, be sure to check out the worksheets
and the online
quizzes!
