# STA 4364 HW 2 solution

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Problem 1: (7 points) ISL Chapter 3 Problem 9. This problem involves linear regression of the Auto
dataset from the last homework. The dataset is available on Webcourses in the file Auto.csv. This dataset
is referenced in the text of both Chapters 2 and 3 and you can look at the information there as a guideline.
One important thing to note is that you should omit rows with missing entries using the na.omit() function
in R or the or the df.dropna() function in pandas, where df is your dataframe. (Note: In general, omitting
NA values might bias your results. But data imputation can be involved so for now we will just remove
missing entries).
Problem 2: (7 points) Analyze the Air Quality dataset provided in Webcourses (file is AirQuality.csv).
This data was collected hourly in Beijing from January 1, 2010 to December 31, 2014. We are interested in
PM2.5 concentration as the response variable. The dataset attributes are:
• No: row number
• year: year of data in this row
• month: month of data in this row
• day: day of data in this row
• hour: hour of data in this row
• pm2.5: PM2.5 concentration (ug/m3
)
• DEWP: Dew Point
• TEMP: Temperature
• PRES: Pressure
• cbwd: Combined wind direction (cv is SW)
• Iws: Cumulated wind speed (m/s)
• Is: Cumulated hours of snow
• Ir: Cumulated hours of rain
Analyze the data using the following steps:
(a) Prepare the dataset: load the data, omit rows with NA entries. Plot the histogram of PM2.5. Since this
variable is clustered around 0, it’s best to take a log transformation to make the data more symmetric
and unimodal. Remove 0 entries for PM2.5 to avoid −∞ values when doing the log transform (there
are only 2 observations with PM2.5 of 0 so this is not a problem. In general care should be taken with
0 entries when doing log transforms to avoid taking the log of 0).
(b) Do an exploratory analysis on the data. Make plots or perform calculations to investigate the following
questions:
• Does pollution appear to be increasing, decreasing, or neither over time?
• Are certain months, days of the month, or hours of the day, associated with greater pollution?
• Are environmental factors associated with greater or less pollution?
• Are there relations between the environmental factors?
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(c) Perform a transformation on month, day, and hour that is appropriate for linear regression with cyclic
variables:
x 7→ (cos(2πx/τ ),sin(2πx/τ ))
where τ = 12, 30, or 24 respectively. Remove the original coding of the cyclic variables from your
dataset. Then split the data into a training and validation set.
(d) Create a linear model predicting PM2.5 using all other features (including No, how should this coefficient be interpreted?). What do you notice about the significance of the coefficients? Why is this
happening? Are these significance values meaningful?
(e) Narrow down the predictors to a group of about 4 to 6 predictors that are most meaningful. Possible
tools for doing this are doing univariate regression with each predictor, or multivariate regression with
smaller number of observations selected randomly, or forward/backward regression with increasing or
decreasing numbers of coefficients (see ISL Section 3.2.2), or a combination of these methods. Do a
multivariate regression on PM2.5 using your smaller set of predictors. How does the R2 value compare
to the full regression from the previous part?
(f) Calculate the mean-square-error (MSE) on your validation set for a model with all predictors and a
model with your reduced set of chosen predictors:
MSE(βˆ) = 1
nval
X
(Xi,Yi)∈Dval
(Yi − X
|
i βˆ)
2
where Dval represents the pairs (Xi
, Yi) in your validation set and nval is the number of validation
observations. Interpret your model by revisiting the questions in Part (b).
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