When is a parameter that reflects visual function abnormal? When is a change in some aspect of visual function meaningful, and when is it just a random chance variation? When is a therapeutic intervention helpful in slowing the progression of disease? When might an intervention be harmful? These are just some of the areas in which statistical analyses can be extremely helpful and informative. In this chapter, basic issues of experimental design and statistical analysis are discussed. Detailed discussion of a variety of statistical issues is beyond the scope of this chapter. Whenever possible, the reader will be referred to primary sources for expanded discussion.