In A level biology exams there is a large focus on data interpretation questions. What is the best way to approach this style of question?

Often this particular style of question ranks amongst the most poorly answered by students. However, it is often very easy to pick up marks without any knowledge of the subject matter whatsoever. The first step should always be to look at what particular information the question wants you to provide wether this be comparing two separate data points or recognising a trend in a data set. It can be very easy to fall into the trap of describing every part of the graph in detail however in an exam situation this can be detrimental as it is essentially a waste of time as it will provide no extra marks for the time investment required. often it is an excellent idea to check the amount of marks available for a question to gather an idea of how much information to provide. When describing the data itself it really is as simple as saying what you see, taking care to note significant differences in the data sets as demonstrated by standard deviation bars and that a correlation shown between two variables does not equal causation. 

In addition to interpreting the data a frequent follow up question will pertain to the validity of the experiment that was carried out to obtain the information presented in the graph. Again, these questions often follow trends that become evident after taking several practice papers. Often the information will be provided in the question so re-read the flavour text taking particular note of any mention of control variables or sample sizes. Make sure you do not simply regurgitate the information it is important to either explain why the particular controls where implemented or to critique the study for what variables it failed to control and perhaps suggest what could have been done differently.