Explain how random assignment is associated with internal ...

[mckennatimberlake]

Explain how random assignment is associated with internal validity and random selection is associated with external validity.
 
  What will be an ideal response?

Question 2

What are some difficulties that scientists have had in categorizing people by race and in defining race?
 
  What will be an ideal response?

[recede]

Answer to Question 1

Random assignment of participants to groups is useful for creating equivalent groups. Thus, differences at the end of the study are likely to be due to differences in the level of the IV. Random selection provides for representative samples because there are no biases that systematically include one group of people or exclude another group.

Answer to Question 2

a. Skin color has often been a means by which people are racially categorized. This is unreliable because sometimes people with darker skin will be considered white while people with lighter skin will be considered black.
b. Race is seem as involving discrete, mutually exclusive categories; a person falls in one and only one category. In reality, the characteristics associated with the categories fall on a continuum, so the cutoff points are arbitrary.
c. Scientists have not reached consensus about what racial categories there might actually be. There is no set number of racial categories that scientists agree on.
d. Geographical criteria for race is unreliable because there are many people whose birthplace is not a reliable indicator of their ancestry, which is likely to be mixed in any case.
e. When scientists did create racial categories, they were based on subjective, nonscientific criteria.