I'm not sure the context of the question but from the available answers "the last straw" seems to apply to that paragraph.
Answer: Variables that need to be included or excluded are defined by the research being done and its limitations also.
OR Inclusion and exclusion criteria may include factors such as age, sex, race, ethnicity, type and stage of disease, the subject's previous treatment history, and the presence or absence (as in the case of the “healthy” or “control” subject) of other medical, psychosocial, or emotional conditions.
Little Lamb, who made thee? Dost thou know who made thee? Little Lamb, I'll tell thee, Little Lamb, God bless thee! Each appear twice in the poem. These help the tone because it sounds childish and innocent.
