# Chapter 12 Finding the best model of gross state product

The following data analysis workflow shows off drake’s ability to generate lots of reproducibly-tracked tasks with ease. The same technique would be cumbersome, even intractable, with GNU Make.

## 12.1 Get the code.

Write the code files to your workspace.

drake_example("gsp")

The new gsp folder now includes a file structure of a serious drake project, plus an interactive-tutorial.R to narrate the example. The code is also online here.

## 12.2 Objective and methods

The goal is to search for factors closely associated with the productivity of states in the USA around the 1970s and 1980s. For the sake of simplicity, we use gross state product as a metric of productivity, and we restrict ourselves to multiple linear regression models with three variables. For each of the 84 possible models, we fit the data and then evaluate the root mean squared prediction error (RMSPE).

\begin{aligned} \text{RMSPE} = \sqrt{(\text{y} - \widehat{y})^T(y - \widehat{y})} \end{aligned} Here, $$y$$ is the vector of observed gross state products in the data, and $$\widehat{y}$$ is the vector of predicted gross state products under one of the models. We take the best variables to be the triplet in the model with the lowest RMSPE.

## 12.3 Data

The Produc dataset from the Ecdat package contains data on the Gross State Product from 1970 to 1986. Each row is a single observation on a single state for a single year. The dataset has the following variables as columns. See the references later in this report for more details.

• gsp: gross state product.
• state: the state.
• year: the year.
• pcap: private capital stock.
• hwy: highway and streets.
• water: water and sewer facilities.
• util: other public buildings and structures.
• pc: public capital.
• emp: labor input measured by the employment in non-agricultural payrolls.
• unemp: state unemployment rate.
library(Ecdat)
data(Produc)
head(Produc)

## 12.4 Analysis

First, we load the required packages. drake is aware of all the packages you load with library() or require().

library(biglm) # lightweight models, easier to store than with lm()
library(drake)
library(Ecdat) # econometrics datasets
library(ggplot2)
library(knitr)
library(purrr)
library(tidyverse)

Next, we construct our plan. The following code uses drake’s special new language for generating plans (learn more here).

predictors <- setdiff(colnames(Produc), "gsp")

# We will try all combinations of three covariates.
combos <- combn(predictors, 3) %>%
t() %>%
as.data.frame(stringsAsFactors = FALSE) %>%
setNames(c("x1", "x2", "x3"))

# We need to list each covariate as a symbol.
for (col in colnames(combos)) {
combos[[col]] <- rlang::syms(combos[[col]])
}

# Requires drake >= 7.0.0 or the development version
# at github.com/ropensci/drake.
# Install with remotes::install_github("ropensci/drake").
plan <- drake_plan(
model = target(
biglm(gsp ~ x1 + x2 + x3, data = Ecdat::Produc),
transform = map(.data = !!combos) # Remember the bang-bang!!
),
rmspe_i = target(
get_rmspe(model, Ecdat::Produc),
transform = map(model)
),
rmspe = target(
bind_rows(rmspe_i, .id = "model"),
transform = combine(rmspe_i)
),
plot = ggsave(
filename = file_out("rmspe.pdf"),
plot = plot_rmspe(rmspe),
width = 8,
height = 8
),
report = knit(knitr_in("report.Rmd"), file_out("report.md"), quiet = TRUE)
)

plan

We also need to define functions for summaries and plots.

get_rmspe <- function(model_fit, data){
y <- data$gsp yhat <- as.numeric(predict(model_fit, newdata = data)) terms <- attr(model_fit$terms, "term.labels")
tibble(
rmspe = sqrt(mean((y - yhat)^2)), # nolint
X1 = terms[1],
X2 = terms[2],
X3 = terms[3]
)
}

plot_rmspe <- function(rmspe){
ggplot(rmspe) +
geom_histogram(aes(x = rmspe), bins = 15)
}

We have a report.Rmd file to summarize our results at the end.

drake_example("gsp")
file.copy(from = "gsp/report.Rmd", to = ".", overwrite = TRUE)

We can inspect the project before we run it.

vis_drake_graph(plan)

Now, we can run the project.

make(plan, verbose = 0L)

## 12.5 Results

Here are the root mean squared prediction errors of all the models.

results <- readd(rmspe)
library(ggplot2)
plot_rmspe(rmspe = results)

And here are the best models. The best variables are in the top row under X1, X2, and X3.

head(results[order(results\$rmspe, decreasing = FALSE), ])

## 12.6 Comparison with GNU Make

If we were using Make instead of drake with the same set of targets, the analogous Makefile would look something like this pseudo-code sketch.

models = model_state_year_pcap.rds model_state_year_hwy.rds ... # 84 of these

model_%
Rscript -e 'saveRDS(lm(...), ...)'

rmspe_%: model_%
Rscript -e 'saveRDS(get_rmspe(...), ...)'

rmspe.rds: rmspe_%
Rscript -e 'saveRDS(dplyr::bind_rows(...), ...)'

rmspe.pdf: rmspe.rds

report.md: report.Rmd
Rscript -e 'knitr::knit("report.Rmd")'


There are three main disadvantages to this approach.

1. Every target requires a new call to Rscript, which means that more time is spent initializing R sessions than doing the actual work.
2. The user must micromanage nearly one hundred output files (in this case, *.rds files), which is cumbersome, messy, and inconvenient. drake, on the other hand, automatically manages storage using a storr cache.
3. The user needs to write the names of the 84 models near the top of the Makefile, which is less convenient than maintaining a data frame in R.

## 12.7 References

• Baltagi, Badi H (2003). Econometric analysis of panel data, John Wiley and sons, http://www.wiley.com/legacy/wileychi/baltagi/.
• Baltagi, B. H. and N. Pinnoi (1995). “Public capital stock and state productivity growth: further evidence”, Empirical Economics, 20, 351-359.
• Munnell, A. (1990). “Why has productivity growth declined? Productivity and public investment”", New England Economic Review, 3-22.
• Yves Croissant (2016). Ecdat: Data Sets for Econometrics. R package version 0.3-1. https://CRAN.R-project.org/package=Ecdat.