When studying rates there are a number of different observations that can be made to monitor the rate of a reaction. Because we are looking at a mathematical relationship between a change in a quantity of reactant or product against time we need to collect accurate numerical data.

Looking at the picture above which ones could provide numerical data?

All of the above could be used to measure rate. Picture A: weight loss vs time. Picture B: use spectrometry to measure the appearance of coloured product. Picture C: bubble counting vs time. Picture D: collecting the volume of gas evolved vs time. The challenge for students here is their lack of familiarity with the variety of techniques available to gather appropriate data.