cs-5821/hw1/answers

1. For each of parts (a) through (d), indicate whether we would generally
   expect the performance of a flexible statistical learning method to be
   better or worse than an inflexible method. Justify your answer.

(a) The sample size n is extremely large, and the number of predic- tors p
is small.

    This seems to still depend on how the data are distributed, but
    generally, I would say a less flexible method will perform better here,
    given that we have a large number of observations to average over.

(b) The number of predictors p is extremely large, and the number of
observations n is small.

    We might want a more flexible method in this case, since the data are
    sparse and we want a model that responds smoothly to possible large
    changes along and across predictors.

(c) The relationship between the predictors and response is highly non-
linear.

    A more-flexible model will clearly be expected to have better
    performance here, as it will reflect the non-linear nature of the real
    function.

(d) The variance of the error terms is extremely high.

    A less-flexible function will likely respond better here, because the
    bias-variance trade-off is concerned with nuanced differences that are
    overwhelmed in a high-ε situation. The variance of f̂ and the bias of
    f̂ are insignificant compared to the variance of the error ε, so we
    don't gain predictability by attempting to reduce them.


2. Explain whether each scenario is a classification or regression problem,
   and indicate whether we are most interested in inference or prediction.
   Finally, provide n and p.

(a) We collect a set of data on the top 500 firms in the US. For each firm
we record profit, number of employees, industry and the CEO salary. We are
interested in understanding which factors affect CEO salary.

    p = 4
    n = 500
    This is a regression problem, as we're predicting
    numerical values using numerical values. Prediction is interesting
    here, because we want to be able to predict CEO salary as a function of
    the predictors we find significant of the 4 available.

(b) We are considering launching a new product and wish to know whether it
will be a success or a failure. We collect data on 20 similar products that
were previously launched. For each prod- uct we have recorded whether it
was a success or failure, price charged for the product, marketing budget,
competition price, and ten other variables.

    p=14
    n=20
    Another prediction problem, because we're interested in a
    predicted outcome -- success or failure -- as a function of the various
    predictors. This could be considered semi-categorical, since at least
    one predictor has a classification nature, but I would say it is a
    classification problem because the goal is to predict a class: failure
    or success.

(c) We are interesting in predicting the % change in the US dollar in
relation to the weekly changes in the world stock markets. Hence we collect
weekly data for all of 2012. For each week we record the % change in the
dollar, the % change in the US market, the % change in the British market,
and the % change in the German market.

    n=52
    p=4
    A clear regression setting, but this is an inference problem,
    not a prediction problem. With inference, we have a starting place and
    attempt to predict the change in a variable as a function of other
    observed rates: in this case, we have a known US dollar price, and we
    want to predict how it will change given rate shifts in other markets,
    so inference clearly applies.



4. You will now think of some real-life applications for statistical
   learning.

(a) Describe three real-life applications in which classification might be
useful. Describe the response, as well as the predictors. Is the goal of
each application inference or prediction? Explain your answer.

    Image identification. The predictors could be things like "distribution
    of greyscale intensity", "distribution of colors", and any number of
    clever things I'm sure machine learning professionals have thought up.
    The response is the most probably classification. This is a prediction.

    Galactic classification. Really this is very similar to general image
    identification, but we classify galaxies using very specific spectral
    bands for the predictors that involve light intensity, but then we also
    look at how strong particular spikes or dips in the spectrum are, so we
    might have predictors for "emission line strength" for several spectral
    features. The response is the most likely galactic classification. This
    is a prediction.

    Speech recognition. The predictors would perhaps be the audio spectrum,
    with the response being the word the audio spectrum corresponds to.
    This would predict the most likely word for the audio received.

(b) Describe three real-life applications in which regression might be
useful. Describe the response, as well as the predictors. Is the goal of
each application inference or prediction? Explain your answer.

    Marketing data is obvious. The predictor is perhaps how much was spent
    on a certain type of marketing, or a few types of marketing -- this is
    now sounding like the example from the book. The response is an amount
    sold for the same fiscal period. You could use inference or prediction
    here: inference to how how many addition sales you might add by
    spending marketing funds, or prediction by asking just "how many sales
    did we see when we spent X amount on marketing?"

    I want to try to use this for my project: understanding the time delay,
    or reverberation, of a dynamic spectral feature compared against a
    similarly dynamic reference feature. 2 predictors, line-of-sight
    velocity and time delay, give a response of light intensity. Our task
    is to predict the light intensity as a function of these predictors.
    This is actually a vanguard question in astrophysics, and I'll bet
    somebody is already trying to do this!

    Maybe something municipal. I could predict the taxable income of a city
    based on a number of predictors, like availability of mass transit or
    highways, demographics, resources, distance to neighbouring cities, and
    all kinds of things, then the response would continue to just be
    taxable income given all of these inputs. Perhaps it would be good to
    consider an inference questions here, for example: how would my city's
    taxable change if I increased the availability of public transit?


(c) Describe three real-life applications in which cluster analysis might
be useful.

    Categorizing star type by spectral band strengths.

    Plant and animal species identification.

    Tracking objects in sensor data.




9. This exercise involves the Auto data set studied in the lab. Make sure
   that the missing values have been removed from the data.

(a) Which of the predictors are quantitative, and which are quali- tative?

    mpg, horsepower, weight, acceleration, and displacement are all clearly
    quantitative.

    cylinders I think is arguably qualitative because each number of
    cylinders defines a somewhat broad class of vehicles. For the years,
    the same argument might apply: each year is a class of vehicles. The
    origin is clearly qualitative, and so is name.

(b) What is the range of each quantitative predictor? You can an- swer this
using the range() function.

    $mpg
    [1]  9.0 46.6

    $cylinders
    [1] 3 8

    $displacement
    [1]  68 455

    $horsepower
    [1]  46 230

    $weight
    [1] 1613 5140

    $acceleration
    [1]  8.0 24.8

    $year
    [1] 70 82

(c) What is the mean and standard deviation of each quantitative predictor?

    $mpg
       mu     sigma 
    23.445918  7.805007 

    $cylinders
          mu    sigma 
    5.471939 1.705783 

    $displacement
        mu   sigma 
    194.412 104.644 

    $horsepower
        mu     sigma 
    104.46939  38.49116 

    $weight
        mu     sigma 
    2977.5842  849.4026 

    $acceleration
        mu     sigma 
    15.541327  2.758864 

    $year
        mu     sigma 
    75.979592  3.683737

(d) Now remove the 10th through 85th observations. What is the range, mean,
and standard deviation of each predictor in the subset of the data that
remains?
    
    $mpg
           mu     sigma 
    24.404430  7.867283 

    $cylinders
          mu    sigma 
    5.373418 1.654179 

    $displacement
        mu     sigma 
    187.24051  99.67837 

    $horsepower
        mu     sigma 
    100.72152  35.70885 

    $weight
        mu     sigma 
    2935.9715  811.3002 

    $acceleration
        mu     sigma 
    15.726899  2.693721 

    $year
        mu     sigma 
    77.145570  3.106217 
    

    I've now changed my mind and say that both year and cylinders are
    quantitative, since there is plenty of sense about talking about the
    mean and std in those predictors for this set of data.

(e) Using the full data set, investigate the predictors graphically, using
scatterplots or other tools of your choice. Create some plots highlighting
the relationships among the predictors. Comment on your findings.

(f) Suppose that we wish to predict gas mileage ( mpg ) on the basis of the
other variables. Do your plots suggest that any of the other variables
might be useful in predicting mpg ? Justify your answer.