This must include Higher level knowledge. Additional science that is not on the syllabus may be used so long as it is of an equivalent standard, or higher. Many people look in old 'O' Level text books or basic 'A' Level text books. Although it is possible to produce original ideas, most of your investigations are not original. It is much easier to look things up, which saves you time, than it is to reinvent the wheel.
Don't forget to use the correct units for values and calculated values. If you are using reference values from data books, or equations, these will normally be given in SI units. (International Standard units) You may have to convert your values to SI units if you want to do any comparisons.
You must have reliable evidence, and you must have had to do some sort of processing. This could be further calculations, measuring gradients, etc. to test a relationship. Science knowledge must be used to give an explanation for the conclusions you can draw from your evidence. Make sure that you state clearly over what range of values your conclusions are valid. It is not uncommon for trends or patterns in your evidence to change at large and/or small values.
This is the simplest of all, the graph will be a straight line. If it also passes through the origin the two variables are directly proportional; y = mx.
A graph of the two variables is a curve. You must draw a second graph to test the relationship. e.g. If you think there is an inverse relationship then y=(m/x)+c. You must first calculate 1/x, then plot y against 1/x. If you now get a straight line you know the relationship is true.
Common relationships to test are; 1/x , 1/x2 , x2 , x3 , square root(x)
Whatever you do, don't just put "my results prove my prediction correct" and expect to get high marks! You need to explain how well your evidence agrees. You must consider the accuracy of your evidence (you have already decided it is reliable). You must discuss where your evidence does not match your prediction and provide (possible) scientific reasons why this might be so. There is no need to repeat yourself if you have already covered this for strand A.8a.