I'm working through a logistic regression example from the lab on logistic regression in Intro to Statistical Learning. When they try to test how accurate their model is they do,
glm.pred[glm.probs >.5] = "Up"
Essentially they are asking whether the predicted probability of a market increase is greater than or less than 0.5. But how did they choose the number 0.5? If there is another situation where probabilities are much lower for each prediciton, do we replace 0.5 with the