Final answer:
David Wechsler added a performance scale to measure nonverbal skills and rule out biases in intelligence testing.
Step-by-step explanation:
The psychologist who added a performance scale in an attempt to measure nonverbal skills and rule out other cultural or educational biases was David Wechsler.
He developed the Wechsler-Bellevue Intelligence Scale, which later became the Wechsler Adult Intelligence Scale (WAIS), one of the most widely used IQ tests today.
This test combines verbal and nonverbal subtests to assess a person's overall intelligence and ability to act purposefully, think rationally, and deal effectively with their environment.