 Now the first thing we're going to go through is the uncertainties in your measurements. You might recall that Joe measured his height as being six roughs, the height of six Scottish terriers. Now I don't have Scottish terriers at home and I don't think I'd be able to get six of them to stand on top of each other. So in my family we've always measured heights by making the person stand up against a door frame, resting a ruler on top of their head, marking the door frame and then getting out a measuring tape to measure from the floor to the mark. So let's pretend I've just measured my door to this way and the number I read off the measuring tape is 139.6 centimeters. The first type of uncertainty in the measurement is dictated by the fineness of the divisions on the ruler. This measuring tape has lines spaced one millimeter apart and at the very best I can read off a measurement to within about half a millimeter. So I should record the height as 139.60 centimeters plus or minus 0.05 centimeters. This specifies the finite precision of your measuring instrument. Now in general for measurements using an analog scale such as a ruler, a dial or a clock, the uncertainty in the reading is half the smallest division of the scale. But the uncertainty in your reading or recording device is not the only way your measurement might be different from the best or true value. And we'll talk about different types of error next.