An observational study is an empirical investigation of the effects of treatments, policies or exposures. It differs from an experiment in that the investigator cannot control the assignment of... This description may be from another edition of this product.
ThriftBooks sells millions of used books at the lowest everyday prices. We personally assess every book's quality and offer rare, out-of-print treasures. We deliver the joy of reading in recyclable packaging with free standard shipping on US orders over $15. ThriftBooks.com. Read more. Spend less.