Combinations may sometimes be fixed, e.g. by taking an average, or a type of a parameterized combination like a weighted linear combination as a principal component analysis;see also [12, 56, 59].

• 组合有时可能是用固定的方法，如通过取平均，或者参数化的组合，象用于主成份分析的带权值的线性组合，参见[12,56,59]。