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1 : Why do we need logistic regression?Can't we just use linear regression to predict the results by setting some threshold?
A : Because the results are far from perfect. Assume we pick the threshold as 0.5
The examples of graph_00 works perfectly, but the prediction is very easy to break if we add new sample into the training set as graph_01 show.
B : the hypothesis function of linear regression can greater than 1 or less than zero.
2 : What is the model of logistic regression?
The course use sigmoid function as the model, the output range of sigmoid function is [0, 1], this is a neat feature for classification.
The output of the hypothesis is same as