After some break, I came back and would like to suggest you reading about biostatistics. Even though in biology olympiads this comprises only a small part of testing, however, you should know t-test and Chi-square very very well.
To begin with, bellow you will find a comprehensive textbook of biological statistics.
Also, take a look at great notes here:
A good summarize on P-VALUES AND HYPOTHESIS TESTING.
Finally, the last manual.
Keep in mind that the majority of this information is not needed during the IBO but an understanding of these concepts is of paramount importance. Take a look on Part 2 for chi square practice problems.
Big thanks to http://udel.edu/~mcdonald/HandbookBioStat.pdf, http://www.shef.ac.uk/polopoly_fs/1.104345!/file/Scope_tutorial_manual.pdf and www.haverford.edu/math/lbutler/glossary/p_values.doc.