This chapter discusses variations in dimension and data. It considers types of data from high-dimensional and lower dimension. Agenome-wide association (GWA) study is a genetic association study in which the genotypes of hundreds of thousands of single-nucleotide polymorphism (SNPs) are measured. Inquantitative-trait loci (QTL) mapping studies, genotype data on alleles at locations in the genome called loci and phenotype data on quantitative (continuous) traits such as weight are used to test for each locus the null hypothesis that it is not linked to the locus of a gene that influences the phenotype. Two loci are said to be linked if inheriting an allele at one locus happens more frequently if an allele at a linked locus is inherited. The chapter shows how to analyze data of a reduced dimension using a method that applies to large dimension. When the dimension is too small for methods designed for high-dimension data, special methods are needed.