An economist compiled data on productivity improvements last year for a sample of firms producing electronic computing equipment. the firms were classified according to the level of their average expenditures for research and development in the past three years (low, moderate, high). a. is this study experimental, observational, or mixed experimental and observational? why? b. identify all factors, factor levels, and factor-level combinations. c. what type of study design is being implemented here? d. what is the basic unit of study?