For each stage of the experiment. Note the units of measurement you are using for these values. Usually, you choose one independent variable at a time and observe its effect on one or more dependent variables. That means that you are supposed to concentrate very specifically on what it is you are supposed to accomplish in the experimental procedure itself. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise.
Precision is independent of accuracy.
You can be very precise but inaccurate, as described above. You can also be accurate but imprecise. Instead of focusing just on the specific actions of the experimental procedure, purpose looks at the experimental procedure within the learning context. The point is to show how what you are doing in the lab is related to that learning context. This is important because learning something is the overall purpose of the lab.