# R logistic regression optimal cut point

I am working on a dataset that has 300+ predictors and the dependent variables is very imbalanced (99:1). I need to have a prediction accuracy to show to my client.Here is my analytical process.

1. clean data: remove incomplete columns and rows, then I have 80% of rows remaining and 100+ predictors.
2. use LASSO: use LASSO with logistic regression to generate the model (by setting up train and testing sets). Then I have problem finding the best cut points. Below is the accuracy stats for the prediction in testing set if I set cut point as 50%:

 pred 0 1 0 825 36 1 23 43 

The prediction accuracy is too low and I am wondering if it could be improved by choosing different cut points.

Appreciate any helps and suggestions. Thanks.

Not sure accuracy is a useful term here.

• Note that the ratio of FP to FN is 36/23 with is 1.5. Fairly close to 1.

• As you decrease the threshold to below 50% you are going to increase your TP at the expense of increasing your FP. The cost ratio of FP/FN will increase.

• If you increase your threshold to above 50%, your FP will decrease and your cost ratio of FP/FN will decrease to below 1.

The question is how costly are false negative estimates compared to false positive estimates? Once you have a sense of that ratio you can then set you probability cut-point. Often with rare events one is willing to endure a fair number of false positives and the ratio is >1. Usually a guesstimate at this ratio if fairly good starting point, but some domain knowledge is needed.

Or if you don't want to make a decision, a ROC curve is one way to present the information.

• thanks for your answer, which is very helpful. Since I don't have much industry experience, I choose to use ROC curve. Inspired by this post stackoverflow.com/questions/16347507/… – Z. Zhang Jan 13 '16 at 20:05