Gabriele Lanaro

11 Jul 2015

Room Prices Analysis (Part 3): Natural Language Modeling and Feature Selection in Python.

After looking on how to scrape data, clean it and extract geographical information, we are ready to begin the modeling stage.

In the modeling stage we work to get the answers we need or to build the actual product. For the room prices analysis we are mostly interested in which variables are correlated with high prices so that we can have a better understanding of the market. In other words we want, given the data, to make inferences about the underlying process that generated it.

Alternatively we may also be interested in predicting future prices. The method choice is going to be dependent on the question we want to answer.

Exploratory data analysis

First of all we’re going to do some exploration. The first step is to load the (cleaned) data into our favorite tool, pandas.

Each entry in our dataset corresponds to a post scraped, the data is stored in json format and contains a series of columns with various features. A good thing to do is to use the method DataFrame.head() or to check the attribute DataFrame.columns. The following table represents a subset of the cleaned data (the rest is omitted for brevity), followed by the full list of columns.

title latitude longitude cats neigh updated_on
1 furnished room for rent available april 1 m... 49.264521 -123.098803 NaN Mount Pleasant 2015-03-05 11:34:56
10 looking for 2 students apr 1 49.281699 -123.133284 NaN West End 2015-03-11 19:37:59
100 nice cosy room one block from the beach k... 49.270175 -123.167798 NaN Kitsilano 2015-03-04 11:21:14
1000 furnished room bathroom in a 2 bed 2bath yal... 49.275445 -123.124949 NaN Downtown 2015-03-05 23:43:24
10000 1 bedroom in 2 bedroom condo for rent 49.264817 -123.156555 NaN Kitsilano 2015-02-28 13:25:08
>>> van_data.columns

Index([u'available_on', u'bedrooms', u'cats', u'city', u'condo', u'content', u'dogs', u'furnished', u'house', u'latitude', u'laundry', u'location', u'longitude', u'neigh', u'parsed_on', u'post_id', u'price', u'size_ft2', u'title', u'updated_on'], dtype='object')

As we can see there is quite a bit of predictors, therefore we may want to run some summaries to check what’s going on. This is done by DataFrame.describe() that will display general statistics for the numerical variables. As you can see some of the predictors are present only for very few posts furnished counts only 21 posts, while the condo variable is not useful as its count is zero.

bedrooms cats condo dogs furnished house latitude laundry longitude price size_ft2
count 92.000000 1014 0 1027 21 19 15013.000000 18276 15013.000000 15438.000000 5089.000000
mean 1.010870 1 NaN 1 1 1 49.257344 1 -123.093584 654.075657 869.664767
std 0.791362 0 NaN 0 0 0 0.071436 0 0.509354 189.294457 4101.154188
min 0.000000 1 NaN 1 1 1 43.652228 1 -124.520477 300.000000 1.000000
25% 1.000000 1 NaN 1 1 1 49.240830 1 -123.132071 500.000000 189.000000
50% 1.000000 1 NaN 1 1 1 49.261026 1 -123.108394 600.000000 740.000000
75% 1.000000 1 NaN 1 1 1 49.278292 1 -123.063698 750.000000 1000.000000
max 4.000000 1 NaN 1 1 1 49.823690 1 -79.380315 1500.000000 145085.000000

Now that we have a feel about the quantities we’re dealing with, we want to start exploring the data graphically. Plots are pretty much the best way you have to capture the nature and relationship between variables. In the following plot we use an histogram to display the distribution of the variable price.

Price histogram

In this case the distribution is quite skewed with a tail at high prices, and a median of about $600. While this is already a great deal of information, we can definitely dig deeper and investigate how price is affected by other measurements.

Density plots (or kernel density estimation) are a great alternative to histograms.

While histograms and density plots are very informative for single variables, boxplots let us compare several distributions at a glance. The following code groups the price by neighbourhood and displays a boxplot graph. The little rectangles in the boxplots span a range between the first and third quartile (50% of the data) and the vertical line inside of each rectangle is the median, the additional lines (whiskers) are set at a distance of 1.5 times the interquartile range.

van_data.boxplot('price', by='neigh', figsize=(15, 10), rot=90)

Price by neighbourhood

While boxplots can’t tell us how the distribution looks like, they’re still useful to look at the spread and skewness of the distributions. For example, we can see how price varies sensibly with the neighbourhood, and how some neighbourhoods have a much larger price range than others.

If you think you want more than box plots, then check violin plots with the seaborn library.

Simple natural language modeling, the bag-of-words

While we got some sparse features from the postings we still have access to a great deal of information: the content of the postings. Understanding natural language (and unstructured data in general) is quite a challenge but, at least for our purposes, it can be effectively addressed with a simple yet popular method, the bag of words.

The idea is to transform the posting text in a vector where each element represent a word. If the word is present, the vector element is set to 1, otherwise it is set to 0.

Bag of words

An even better approach (and the one I used) is to employ bigrams, that is to take into account pairs of words instead of single words. For example the sentence.

python is a dynamic language

Would produce the following bigrams:

Additionally, it is good practice to remove the so called stop words — words that are very frequent but carry very little specific information, for example the, is, it etc.

This bag-of-digrams operation can be easily accomplished with the library scikit-learn as follows. CountVectorizer takes the following arguments:

The method fit_transform will return the vectorized representation of the content of each post.

# Transform data so that we have no NaN
content_price = van_data[['content', 'price']].dropna()

from sklearn.feature_extraction.text import CountVectorizer
count = CountVectorizer(token_pattern=u'(?u)\\b[a-zA-Z]{2,}\\b',
ngram_range=(1, 2),
stop_words='english',
binary=True)
X = count.fit_transform(content_price.content)

After transforming our input to a bag-of-word (actually a bag of bigrams) representation, we have acquired an enormous amount of features (one for each word in the vocabulary). However, most of those features are very sparse and of little use.

High dimensionality is a big problem and many methods fail or become ineffective when there are a lot of dimensions (see curse of dimensionality), We’ll use some techniques to select the best, most-relevant features.

Feature selection

To think that every single word is important for the price of a room would be naive. Of all those features only few of them have an effect on the price, to extract the important features there are a variety of approaches, I personally prefer the simple ones.

A very simple approach to identify the most relevant features is to run a linear regression using one predictor at a time and to pick only the features that have a (statistically) significant effect on the price. This is already implemented in the package scikit-learn by using the class SelectKBest. The arguments are:

In the following code we select the first 200 features that have the biggest effect for the variable price.

from sklearn.feature_selection import SelectKBest, f_regression
kbest = SelectKBest(score_func=lambda x, y: f_regression(x, y, center=False), k=200)
kbest.fit(X, content_price.price)
important = kbest.get_support(True)

Now that we have a more manageable set of candidates we can fine-tune our selection with better methods.

The Lasso

Lasso is one of my favourite techniques, it is similar to a standard linear regression, where we minimize the sum of squares, but instead of minimizing only the sum of squares, we also add a term to prevent the regression coefficients from assuming extreme values and overfit the data. This process is also called as regularization.

Introducing this extra feat is done by adding an extra term to minimize. In particular, we want that the absolute values of the coefficients stays small:

Where:

is the usual sum-of-squares;

is the sum of the absolute values of the coefficients and is a term chosen by the user that controls how much the coefficients are allowed to grow.

One important caveat of the lasso is that the minimization is able to shrink some coefficients to exactly zero, effectively acting as a filter or variable selection method.

The only problem left is picking a good value for . Such parameter optimization is usually handled by a procedure called cross validation.

Cross Validation

Our objective is to pick a value of that eliminates the useless features and produces the best model possible. But what is the best model possible? It’s a model that is more accurate to predict tomorrow’s, or unseen, data.

Cross validation helps producing some unseen data from the data we already have.

We can split the data in 3 (or more generally k) sets, create the model using the first two sets and we optimize the parameter on the third one. Also, we do the same procedure by selecting each time different sets. A figure illustrates the concept better:

3-fold CV

This procedure can give us an estimate of the error we may get on unseen data and we can use this error to optimize parameters (for example )

The cross-validated Lasso is again conveniently implemented in the library scikit-learn, through the class LassoCV. The following code is going to take quite a long time:

from sklearn.linear_model import LassoCV
lasso = LassoCV(cv=3)
lasso.fit(X[:, important], content_price.price)

After the fit is done we can access the coefficients of the lasso through the attribute lasso.coef_. By looking up words in the CountVectorizer we can retrieve terms along with coefficients, ordered from highest (most positively correlated) to lowest.

for c, i in sorted(zip(lasso.coef_, important), reverse=True):
print "% 20s | coef: %.2f" % (count.get_feature_names()[i], c)

Notice how the words location and email are exactly 0. They were effectively filtered by the lasso.

    contact info  | coef: 83.27
       apartment  | coef: 82.04
         parking  | coef: 71.24
           suite  | coef: 67.44
        bathroom  | coef: 48.54
           great  | coef: 44.49
           fully  | coef: 36.13
         bedroom  | coef: 33.68
             bed  | coef: 31.68
             ubc  | coef: 29.53
            pets  | coef: 24.86
        includes  | coef: 21.18
         private  | coef: 20.80
           floor  | coef: 12.55
         looking  | coef: 12.14
         laundry  | coef: 12.09
       vancouver  | coef: 10.92
     living room  | coef: 10.18
            room  | coef: 9.78
           large  | coef: 8.77
        skytrain  | coef: 6.06
          closet  | coef: 5.75
       available  | coef: 4.58
           quiet  | coef: 2.20
        location  | coef: -0.00
           email  | coef: 0.00
       furnished  | coef: -2.71
           clean  | coef: -4.38
        internet  | coef: -5.98
           month  | coef: -7.30
        downtown  | coef: -9.03
           house  | coef: -10.61
        students  | coef: -12.50
            rent  | coef: -13.28
          street  | coef: -14.14
        included  | coef: -15.17
            walk  | coef: -16.28
           cable  | coef: -19.70
             bus  | coef: -21.83
          living  | coef: -23.42
         contact  | coef: -24.57
           close  | coef: -26.25
         station  | coef: -27.18
       utilities  | coef: -29.06
         student  | coef: -32.03
         smoking  | coef: -33.49
         kitchen  | coef: -37.93
          shared  | coef: -41.05
           share  | coef: -58.98
            info  | coef: -65.55

By analyzing the data we were able to obtain a set of candidate words that correlate with higher or lower price. I think this is an interesting result to look at :).

The Model

Now that we have our hands on the most important features we can construct a simple model. Since we’re interested in inference rather than prediction we want to pick something simple like a linear regression.

For inference we are going to use statsmodels, as it gives a more complete statistical treatment for regression. As you’ve seen before, the Lasso implementation in scikit-learn doesn’t give us any indication on how much should we believe those numbers obtained.

While constructing a model for inference you typically want to know how certain variables affect the outcome in question. A simple linear regression allows us to easily obtaining the effect of some variables excluding (controlling) for others. For example, we can see what the effect of the word apartment, regardless on the neighbourhood.

Here is an example model that calculates the price based on neighbourhood, the presence of the word apartment, and the presence of the word shared:

price = a + b * neighbourhood + c * apartment + d * shared

The model can be implemented using statsmodels as follows. There are quite a few points to notice:

import statsmodels.formula.api as smf
price = content_price.price
neigh = van_data.ix[content_price.index].neigh.fillna('none').values
apartment = X[:, count.vocabulary_['apartment']].toarray().squeeze()
shared = X[:, count.vocabulary_['shared']].toarray().squeeze()

predictor = pd.DataFrame.from_dict({'neigh': neigh, 'apartment': apartment, 'shared': shared, 'price': price})

model = smf.glm('price ~ 1 + C(neigh, Treatment("none")) + apartment + shared', predictor)
fit = model.fit()
fit.summary()
Generalized Linear Model Regression Results
Dep. Variable: price No. Observations: 15438
Model: GLM Df Residuals: 15413
Model Family: Gaussian Df Model: 24
Link Function: identity Scale: 28414.9282432
Method: IRLS Log-Likelihood: -1.0105e+05
Date: Sat, 11 Jul 2015 Deviance: 4.3796e+08
Time: 18:29:12 Pearson chi2: 4.38e+08
No. Iterations: 4
coef std err z P>|z| [95.0% Conf. Int.]
Intercept 625.7286 2.507 249.622 0.000 620.816 630.642
C(neigh, Treatment("none"))[T.Arbutus-Ridge] 102.5775 18.650 5.500 0.000 66.024 139.131
C(neigh, Treatment("none"))[T.Downtown] 199.0859 4.932 40.369 0.000 189.420 208.752
C(neigh, Treatment("none"))[T.Dunbar-Southlands] 79.4633 10.774 7.375 0.000 58.346 100.580
C(neigh, Treatment("none"))[T.Fairview] 151.1972 12.375 12.218 0.000 126.942 175.453
C(neigh, Treatment("none"))[T.Grandview-Woodland] 11.8131 10.386 1.137 0.255 -8.544 32.170
C(neigh, Treatment("none"))[T.Hastings-Sunrise] -59.0837 6.653 -8.880 0.000 -72.124 -46.043
C(neigh, Treatment("none"))[T.Kensington-Cedar Cottage] -55.0974 6.042 -9.119 0.000 -66.940 -43.255
C(neigh, Treatment("none"))[T.Kerrisdale] 40.4121 12.943 3.122 0.002 15.045 65.779
C(neigh, Treatment("none"))[T.Killarney] -53.0172 18.546 -2.859 0.004 -89.366 -16.668
C(neigh, Treatment("none"))[T.Kitsilano] 58.5964 7.112 8.239 0.000 44.657 72.535
C(neigh, Treatment("none"))[T.Marpole] 42.1428 12.543 3.360 0.001 17.560 66.726
C(neigh, Treatment("none"))[T.Mount Pleasant] 27.3563 9.514 2.875 0.004 8.708 46.004
C(neigh, Treatment("none"))[T.Oakridge] -16.4488 13.199 -1.246 0.213 -42.318 9.420
C(neigh, Treatment("none"))[T.Renfrew-Collingwood] -76.9319 5.780 -13.309 0.000 -88.261 -65.603
C(neigh, Treatment("none"))[T.Riley Park] 51.4610 8.902 5.781 0.000 34.014 68.908
C(neigh, Treatment("none"))[T.Shaughnessy] -7.7521 15.181 -0.511 0.610 -37.506 22.002
C(neigh, Treatment("none"))[T.South Cambie] 65.9736 14.485 4.555 0.000 37.584 94.363
C(neigh, Treatment("none"))[T.Strathcona] 25.6578 13.717 1.871 0.061 -1.226 52.542
C(neigh, Treatment("none"))[T.Sunset] 28.3662 7.723 3.673 0.000 13.230 43.502
C(neigh, Treatment("none"))[T.Victoria-Fraserview] -118.8976 10.413 -11.419 0.000 -139.306 -98.489
C(neigh, Treatment("none"))[T.West End] 29.8549 5.488 5.440 0.000 19.099 40.611
C(neigh, Treatment("none"))[T.West Point Grey] 59.6830 12.770 4.674 0.000 34.654 84.712
apartment 57.2537 3.745 15.289 0.000 49.914 64.593
shared -13.6907 2.921 -4.687 0.000 -19.416 -7.965

Cool, how do we interpret those numbers?

Additionally, all this information comes with 95% confidence intervals, that gives us a range in which the true value is supposed to be. For example, given the baseline, being in Downtown with respect to neighbourhood not specified will give you an increase in price that is between $189.420-$208.752 (with 95% confidence).

Other models

There’s quite a bit of models under the sun, here’s a set of improvements that could be of help:

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