Piaget's stage of cognitive development in which mental operations are abstract and hypothetical is:

What percentage of children in the United States has a formal pre-kindergarten experience, such as preschool?

a. 20% b. 40% c. 50% d. 70%

Tied scores often present a problem in distribution-free tests. The most common way to deal with them in a Mann-Whitney test is to

a. flip a coin. b. use a random number table. c. assign tied ranks. d. throw out the tied data.