Libraries

Before you start, as usual we need some libraries.

library(sf)
library(tmap)
library(dplyr)
tmap_mode("view")

The data

Provided you have unpacked this week’s materials to an accessible folder and opened a .Rproj file in the usual way, you should find the datasets by simply running the commands shown below. If that doesn’t seem to work, then download the data from the links provided in the section below. You should find all the data in a subfolder called data, if you unpacked the zip file correctly.

welly <- st_read("data/wellington-base-data.gpkg")

plot(welly, lwd = 0.5, pal = RColorBrewer::brewer.pal(9, "Reds"))

abb <- st_read("data/abb.gpkg")

tm_shape(abb) + 
  tm_dots()

amenities <- st_read("data/amenities.gpkg")

tm_shape(amenities) +
  tm_dots(col = "amenity")

shops <- st_read("data/shops.gpkg")

tm_shape(shops) + 
  tm_dots(col = "shop")

## Warning: Number of levels of the variable "shop" is 115, which is larger than
## max.categories (which is 30), so levels are combined. Set
## tmap_options(max.categories = 115) in the layer function to show all levels.

Organising the data

The idea is to account for the numbers of Airbnb listings per Statistical Area 2 (SA2) using attributes in the base data welly and in the two point datasets amenities and shops.

To do this we need to count the points in the SA2 polygons. We saw how to do this a few weeks ago, but as a reminder, so you know how to count shops and amenities, here’s how to count the Airbnb listings

n_abb_listings <- welly %>%
  st_contains(abb) %>%
  lengths()
n_abb_listings

##   [1] 10 13 11  4  9 12 11  2  5 13  3  2  1  1  4  2 16  5  3 24  6  5  7  3  7
##  [26]  7  2  1  3  8  5  1  1  5  0  2 22  3  3  5  0 18  0  2 12  2  6  7  6 10
##  [51] 14  9 14  7  2 10  3  7  8  2  9  4  8 22  9 19 16  8 10 15 15 17 27 18  7
##  [76] 14 17 20 44 10 10 31  6 47 15 46 46 60 20 21 29 12 17  9 63 25 16 14  8 15
## [101] 23  7 18  8  7 20 23 23  7 29  6  9 12  5  7 15 27 11 12  1 19 27

And this list of counts can then be added to the base data using a mutate operation

welly <- welly %>%
  mutate(n_abb = n_abb_listings)

And then we can map this:

tm_shape(welly) + 
  tm_polygons(col = "n_abb")

Similar operations can be applied to count the amenities and/or shops points.

When you make the model requested for this assignment, an important step is to decide which amenities and which shops in the respective datasets matter. You could include some, none or all of them each represented as a count of how many there are of the respective types in each SA2. For example, it seems likely that cafes and restaurants might be of interest to potential Airbnb renters, but hardware shops may be less relevant.

Adding the kinds of amenities and shops you think are relevant to the base dataset will involved filtering the point data down to only the types you want to keep, and then counting them using an operation like the above, then adding them into the base data. Remember that when you filter data, you need to assign the result of the filter to a new variable to retain the effect of applying the filter, before doing the point-in-polygon counting as above.

When you add new attributes to the dataset, I recommend that you save it to a new file so you don’t lose the work, using the st_write() function.

Building a regression model

Because R is a platform for statistical computing, making regression models (or more generally linear models) is central to what it does. To demonstrate, I an going to walk you through making a model using all the census variables as independent variables to fit the n_abb variable. This is actually a bad idea for reasons we get to a bit later.

The lm function does all the work, all it needs is the equation we want to fit, which we specify as shown below:

m.ages <- lm(n_abb ~ u15 + a15_29 + a30_64 + o65, data = welly)

Nothing seems to have happened, but a model has been made and assigned to the m.ages variable, and we can see what it looks like with the summary function:

summary(m.ages)

## 
## Call:
## lm(formula = n_abb ~ u15 + a15_29 + a30_64 + o65, data = welly)
## 
## Residuals:
##     Min      1Q  Median      3Q     Max 
## -23.149  -5.069  -1.149   4.806  36.625 
## 
## Coefficients:
##             Estimate Std. Error t value Pr(>|t|)    
## (Intercept)  -2.7616     9.4396  -0.293 0.770382    
## u15          -0.8040     0.2133  -3.769 0.000258 ***
## a15_29        0.3687     0.1159   3.182 0.001873 ** 
## a30_64        0.4775     0.1443   3.309 0.001244 ** 
## o65          -0.1602     0.1855  -0.863 0.389634    
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 9.499 on 117 degrees of freedom
## Multiple R-squared:  0.3558, Adjusted R-squared:  0.3337 
## F-statistic: 16.15 on 4 and 117 DF,  p-value: 1.471e-10

Recall that the regression model is effectively an equation predicting the dependent variable. In this case the equation is

\[ \mathrm{n\_abb}=\beta_0+\beta_1\mathrm{u15}+\beta_2\mathrm{a15\_29}+\beta3\mathrm{a30\_64}+\beta_4\mathrm{o65}+\epsilon \]

The \(\beta\) values are the model coefficients, and \(\epsilon\) is an error term (the model is approximate, so includes errors). In the summary view, the model coefficients are shown in the Coefficients table in the Estimate column. The *** designations in this table tell us which of the independent variables are most statistically significant, in this case, it seems like u15 has that honour, but two of the other age groups variables are also significant, albeit less so. Information about the model error is included below the table.

The sign (positive or negative) of the coefficients in the Estimate column tells us the sense of the relationship:

• a positive sign means that when that attribute increases the dependent variable also increases
• a negative sign means the opposite: where that attribute is higher the dependent variable tends to be lower

The values of the coefficients also tell us how much change to expect in the dependent variable for each unit change in the independent variable. For example, for every one point increase in the percentage of population under 15 the model is saying that we expect about 0.8 fewer listings in a census tract. This means that on average a SA2 with (say) 10% aged under 15 would have 4 fewer listings than a SA2 with 5% aged under 15, because it is 5 points higher and \(5\times-0.8040=-4.02\).

Residual mapping

Residuals are the model errors—the variation in the dependent variable that the model does not account for. Mapping residuals can be informative. Model residuals are available from the model we made, m.ages, and can be added to the spatial data as a new attribute to be mapped:

welly <- welly %>%
  mutate(residual = m.ages$residuals)

tm_shape(welly) +
  tm_polygons(col = 'residual', palette = 'RdBu', alpha = 0.65)

## Variable(s) "residual" contains positive and negative values, so midpoint is set to 0. Set midpoint = NA to show the full spectrum of the color palette.

Where the residuals are high (i.e., positive), the model is underestimating the number of listings, that is there are more listings than we might reasonably expect. Where the residuals are low (i.e., negative) the model is overestimating the number of listings, that is there are fewer listings than might be expected.

Across most of the area, the model is not doing terribly but there are a couple of places where it is badly out. The next question asks you to examine these places a bit more closely, using the web map view, for contextual information that might explain the weakness of the model in these places. The best web map for that contextual information is probably the OpenStreetMap layer.

Challenges and interpretations

One of the many difficulties with this model (which wasn’t made with too much thought) is the problem of multicollinearity which means that related variables can mask each other out. Here because the different age group percentages must sum to 100, they are not independent of one another. This can lead to instability in model coefficients. For example, if we remove one of the age groups, unexpected changes happen in the model. To see this here’s a model that leaves out the percentage aged under 15 variable.

m.no_children <- lm(n_abb ~ a15_29 + a30_64 + o65, data = welly)
summary(m.no_children)

## 
## Call:
## lm(formula = n_abb ~ a15_29 + a30_64 + o65, data = welly)
## 
## Residuals:
##     Min      1Q  Median      3Q     Max 
## -22.106  -6.758  -2.404   4.994  40.271 
## 
## Coefficients:
##              Estimate Std. Error t value Pr(>|t|)    
## (Intercept) -17.19805    9.09773  -1.890   0.0612 .  
## a15_29        0.61657    0.10057   6.131 1.19e-08 ***
## a30_64        0.31892    0.14556   2.191   0.0304 *  
## o65           0.02764    0.18843   0.147   0.8836    
## ---
## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
## 
## Residual standard error: 10.02 on 118 degrees of freedom
## Multiple R-squared:  0.2775, Adjusted R-squared:  0.2592 
## F-statistic: 15.11 on 3 and 118 DF,  p-value: 2.195e-08

This model now implies that every percentage point increase in any of the three age groups included will increase the number of Airbnb listings, and also that the 15-29 year old age group has the largest effect, where before the effect of the 30 to 64 year old age group was greater.

This demonstrates how complicated the interrelationships among many attributes in a dataset can be, and how if we control for one factor, it can change the apparent effect of other factors.

It also strongly suggests that the original model including all ages groups is not very reliable! Keep this in mind as you build your own model to complete this assignment.

Finally: build your own model

With the data available, and based on all you have seen so far, make your own model.

You should consider including any of the other variables provided such as total population (pop), the distance to centre (dist_cbd), and (after filtering and counting them) any of the various amenity types or shop types.

You can also experiment with the step() function discussed in lecture here.

When you have made a model you are happy with answer the following question.

Question 1

Question 2

Question 3

Question 4

Submission instructions

This week’s assignment requirement is set out in the questions above.

You should prepare a document completing the analysis and answering the questions, and submit a short report to the dropbox provided on Nuku by the end of the day indicated on the course schedule page.

You can prepare your report using Rmarkdown and knitting the report (this is explained in the first part of this week’s materials), or you can proceed as usual preparing your report in Word and including figures and tables as you usually would and saving out to a PDF. Either is acceptable, but I would encourage you to experiment with knitting an Rmarkdown file to understand the possibilities!