$30.00
The main point of this lab is to scrape data from the Web. We will again generate a literate
report that describes a simple analysis, but we will go back to predicting long trips (a binary
variable) based on trip duration. In addition to fitting a logistic regression as before, we will also
fit a k-nearest neighbours model (see the section at the end of this document).
The Data
The data are again electronic bicycle or scooter trips, but we will access the data from the original
source: data.austintexas.gov, the official City of Austin data portal.
The “Dockless Vehicle Trips” data set that we have been using has its main page here:
https://data.austintexas.gov/Transportation-and-Mobility/Dockless-Vehicle-Trips/7d8e-dm7r
We will use the API that is described here:
https://dev.socrata.com/foundry/data.austintexas.gov/7d8e-dm7r
NOTE: you will need to register for an app token in order to use the API.
The Task
1. Scrape data from the City of Austin data portal and create a data frame with two columns:
trip_distance and trip_duration. You should extract 10,000 scooter trips that occurred
in 2018 (your API request must specify records with year equal to 2018 and vehicle_type
equal to “scooter”).
2. Subset only trips with non-negative distances and durations, log the durations, and create a
new “long trip” variable (where “long” means that the trip distance was greater than 1000m).
3. Fit a logistic regression model to the training data and calculate the accuracy of the model
on the test data.
4. Fit a k-nearest neighbours model and calculate its accuracy. You might need to try a few
different values of k.
5. Produce a plot based on the test data that shows the predictions from the logistic regression
model and the predictions from the k-nearest neighbours model.
The Report
Your submission should consist of a tar ball (as generated by the tar shell tool) that contains an
R Markdown document and a Makefile and a processed version of your R Markdown document,
submitted via Canvas.
You should write your document and your Makefile so that the tar ball can be extracted into
a directory anywhere on one of the virtual machines provided for this course (sc-cer00014-
04.its.auckland.ac.nz or sc-cer00014-05.its.auckland.ac.nz) and the markdown document can be processed just by typing make.
Your report should include:
1
• A description of the data format.
• A description of the method used to scrape the data into R, inclduing an explanation of your
API request.
• A comparison of logistic regression and k-nearest neighbours.
• A conclusion summarising the analysis.
Your report should NOT be longer than 10 pages.
Marks will be lost for:
• Submission is not a tar ball.
• More than 10 pages in the report.
• R Markdown file does not run.
• Section of the report is missing.
• R Markdown file is missing.
• Processed file (pdf or docx or html) is missing.
• Makefile is missing.
• Significantly poor R (or other) code.
k-nearest neighbours
A k-nearest neighbours (classification) model predicts y for a set of test x based on the majority vote from a set of training y, only using the nearest k neighbours, where distance between
neighbours is (by default) euclidean distance between the test x and the training x values.
The k-nearest neighbours model allows for much greater flexibility than a simple logistic regression;
simple logistic regression only allows a single (smooth) transition between two categories, but knearest neighbours allows both sharp transitions and more than one transition between categories.
The danger with this greater flexibility is a higher risk of over-fitting; a trained model may pay
too much attention to details in the training set, which can then lead to more errors in a test set.
The choice of k is critical; ideally, we get a balance between flexibility and generality, so our model
captures more complex trends, but only genuine trends. We typically choose an odd number for
k (to reduce chance of ties).
We can use the knn() function from the class package to generate predictions for a k-nearest
neighbours.
> ## KNN example on randomly generated data
> library(class)
> library(caret)
> set.seed(10)
> x <- sort(runif(1000))
> y <- c(rep(0, 200), sample(0:1, 300, replace=TRUE, prob=c(.4, .6)),
+ sample(0:1, 300, replace=TRUE, prob=c(.6, .4)), rep(1, 200))
> testset <- sort(sample(1:1000, 200))
> trainx <- x[-testset]
> trainy <- y[-testset]
> testx <- x[testset]
> testy <- y[testset]
2
> glmFit <- glm(y ~ x, data.frame(y=trainy, x=trainx), family=”binomial”)
> glmProb <- predict(glmFit, data.frame(x=testx), type=”response”)
> glmPred <- ifelse(glmProb > .5, 1, 0)
> glmDiag <- confusionMatrix(factor(glmPred), factor(testy))
> glmDiag$table
Reference
Prediction 0 1
0 62 30
1 48 60
> glmDiag$overall[“Accuracy”]
Accuracy
0.61
> knnPred <- knn(matrix(trainx, ncol=1),
+ matrix(testx, ncol=1),
+ trainy,
+ k=31, prob=TRUE)
> knnProb <- ifelse(knnPred == 1,
+ attr(knnPred, “prob”),
+ 1 – attr(knnPred, “prob”))
> knnDiag <- confusionMatrix(knnPred, factor(testy))
> knnDiag$table
Reference
Prediction 0 1
0 79 19
1 31 71
> knnDiag$overall[“Accuracy”]
Accuracy
0.75
> par(mar=c(3, 3, 2, 2))
> breaks <- seq(0, 1, .1)
> midbreaks <- breaks[-1] – diff(breaks)/2
> props <- tapply(testy, cut(testx, breaks), mean)
> plot(midbreaks, props, pch=16)
> lines(testx, glmProb, col=”red”)
> lines(testx, knnProb, col=”green”)
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props
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