stratified splitting the data

Question

I have a large data set and like to fit different logistic regression for each City, one of the column in my data. The following 70/30 split works without considering City g

Answer 1

Try createDataPartition from caret package. Its document states: By default, createDataPartition does a stratified random split of the data.

library(caret)
train.index <- createDataPartition(Data$Class, p = .7, list = FALSE)
train <- Data[ train.index,]
test  <- Data[-train.index,]

it can also be used for stratified K-fold like:

ctrl <- trainControl(method = "repeatedcv",
                     repeats = 3,
                     ...)
# when calling train, pass this train control
train(...,
      trControl = ctrl,
      ...)

check out caret document for more details