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5.3. WHEN ADDING VARIABLES HURTS 151variables, body size and neocortex, are correlated across species makes it hard to see theserelationships, unless we statistically account for both.Overthinking: Simulating a masking relationship. Just as with understanding spurious association(page 144), it may help to simulate data in which two meaningful predictors act to mask one another.Suppose again a single outcome, y, and two predictors, x pos and x neg . e predictor x pos is positiveassociated with y, while x neg is negatively associated with y. Furthermore, the two predictors arepositively correlated with one another. Here’s code to produce data meeting these criteria:N

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