That quantity which, when multiplied by, divided by, added too or subtracted from the answer which you got, yields the answer you should have gotten. When you don't know what you are doing, do it neatly. Experiments must be reproduceable. They should fail the same way each time. First draw your curves, then plot your data.
Experience is directly proportional to equipment ruined.
A record of data is essential. It shows you were working. To study a subject best, understand it thoroughly before you start. To do a lab really well, have your report done well in advance. If you can't get the answer in the usual manner, start at the answer and derive the question.