In additive relationship, Z defines a large compositional difference across values of X.



  1. State whether true or false and explain your reasoning.
  2. In additive relationship, Z defines a large compositional difference across values of X. False. The covariate Z is correlated with X and both predict the values of Y.
  3. Field Experiments have lower external validity than do laboratory experiments. True. In dealing with field experiments, a lot of assumptions are made but in laboratory experiments, more details are examined which emphasizes on a higher validity.
  4. Random assignment neutralizes rival explanations. False. Random assignment does not consider a control experiment and in cases where a control experiment is assigned, then the similarity between the control entities and the research entities is not put into consideration.
  5. StateArt simulates diplomatic, military and trade relations among countries.
  6. Rival Explanations
    – Younger students seemed acquire better understanding of the IR subject matter. Younger students are more interested in playing games and thus, they stand a better chance of learning faster due to incorporation of the StateArt.
    – The brighter students understood the IR subject better than the relatively low witted students. Students who are well endowed brain wise are faster learners generally, it is therefore expected that they would have a better understanding.
  7. Improving the study
    – Randomly select the students for the testing. Random selection ensures that all groups of students are covered in the research.
    – Once you draw both the test and control groups, Identify the possible scenarios for rival explanations. This will help in determining the changes to undertake.
    – Making sure that the control and test groups are stochastic duplicates. This helps eliminate rival explanations.
  8. X, Y , Z relationships
  9. Students who smoke (X) earn lower grades (Y) than students who do not smoke.
    Conclusion: Smoking causes poor grades.
    Plausible variable (Z) – Discipline
    Students who smoke are less disciplined. Less disciplined students earn lower grades (Y). As smoking (X) varies, discipline (Z) also varies which makes grade levels (Y) to change.
  10. Expectant mothers who drink bottled water (X) have healthier babies (Y) than do expectant mothers who do not drink bottled water.
    Conclusion: drinking bottled water causes healthier babies to be born.
    Plausible Variable (Z) – Drinking enough water
    Expectant mothers who drink bottled water tend to drink enough amounts of water. Expectant mothers who drink enough amounts of water have healthier babies (Y). As drinking bottled water (X) varies, so does the amounts of water drunk (Z) which makes the health of the babies delivered (Y) to change.
  11. Tea drinkers (X) are more likely to be democrats (Y) than are non-tea drinkers.
    Conclusion: Tea drinking causes people to become democrats.

Plausible Variable (Z) – Race
Most people who drink tea are African Americans. African Americans are more likely to be democrats (Y). As drinking tea (X) varies, so does Race (Z) which in turn makes democrats support (Y) change.

4 A

  1. plausible alternative variable: Level of Education of the individuals
  2. Younger individuals could have a higher level of education than the older individuals. If the young people are more likely to vote democratic than the older people, then, differences in education, not differences in the age, could explain the relationship between the dependent and the independent variable.

4 B

  1. Plausible alternative variable: Border disagreements
  2. Nondemocratic countries could have more border disagreements with the neighboring nations than the democratic countries. If the non democratic countries are more likely to initiate international military conflicts than the democratic nations, then, border disagreements, not the level of democracy in the country, could explain the relationship between the dependent and the independent variable.


  1. Plausible alternative variable: Level of civic education
  2. Nonsoutherners could have a higher level of civic education than the southerners. If the Southerners are less likely to turn out to vote than are nonsoutherners, then the differences in the levels of civic education, not the differences in the geographical locations of the homes, could explain the relationship between the dependent and the independent variable.

Question 5


  1. How Z could produce a spurious relationship between X and Y

Z is the level of education, X stands for the age of the individuals while Y denotes the legalization opinions. By holding the level of education (Z) constant, the age (X) of the individuals is given room to keep on varying. This is done with the objective of establishing the effect of age on the legalization opinions. The results indicate that those who are younger are more likely to favor the legalization of marijuana than those who are older. To this end, the level of education does not have an effect on the legalization opinion of marijuana, thus Z produces a spuriousness relationship between X and Y.






  1. How Z could produce an additive relationship

A combination of the level of education and the age of the individuals could have an impact on the legalization of marijuana opinions. The younger people who have a higher level of education will be more likely to favor legalization than will those who are older and have a low level of education. Thus, the two quantities are seen as having a relationship through addition. Therefore, they produce an additive relationship.


  1. Sketch of a graph indicating an interaction relationship