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.
http://udel.edu/~mcdonald/HandbookBioStat.pdf,
A good summarize on P-VALUES AND HYPOTHESIS TESTING.
http://www.shef.ac.uk/polopoly_fs/1.104345!/file/Scope_tutorial_manual.pdf
Finally, the last manual.
www.haverford.edu/math/lbutler/glossary/p_values.doc
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.
