Piaget’s studies have helped psychologists and educators understand a lot about communicating and teaching children, especially in discovery learning. The others (a, b and d) are all true: he had a very small sample size, used very subjective research techniques (i.e. observation and interviewing), and he failed to consider the environment in his theory. In terms of answer d, 40-60% of college students did not have formal operations thinking and one third of adults did not.