4 Preferences: Utilities, predictions, and simulations

library(tidyverse)
library(mlogit)
library(marginaleffects)
library(parameters)
library(tinytable)
library(scales)
library(ggforce)

stickers <- readRDS("data/processed_data/study_5_sticker.rds")

stickers_indexed <- stickers |> 
  group_by(resp_id, question) |> 
  mutate(choice_id = cur_group_id()) |> 
  ungroup() |> 
  dfidx(
    idx = list(c("choice_id", "resp_id"), "alt"),
    choice = "choice",
    shape = "long"
  )

model_mlogit <- mlogit(
  choice ~ 0 + price + packaging + flavor,
  data = stickers_indexed
)

pars_random <- rep("n", length(model_mlogit$coefficients)) |> 
  setNames(names(model_mlogit$coef))

model_mlogit_hierarchical <- mlogit(
  choice ~ 0 + price + packaging + flavor,
  rpar = pars_random, panel = TRUE, correlation = TRUE,
  data = stickers_indexed
)

4.1 Population-level stuff

4.1.1 Model βs

Parameter	Coefficient	SE	CI_low	CI_high	z	p
price$3	-1.82	0.15	-2.11	-1.53	-12.33	<0.001
price$4	-3.98	0.22	-4.42	-3.54	-17.79	<0.001
packagingPlastic + sticker	1.23	0.12	0.99	1.47	10.00	<0.001
flavorNuts	-3.93	0.23	-4.38	-3.48	-17.06	<0.001

4.2 Individual-level stuff: Part-worth utilities and ratios

	Price			Packaging		Flavor
ID	$2	$3	$4	Paper	Sticker	Chocolate	Nuts
4	0	-2.22	-3.98	0	-4.24	0	-9.76
5	0	-2.55	-5.99	0	-0.19	0	-0.14
6	0	-1.31	-2.87	0	2.67	0	-8.60
7	0	-1.61	-3.34	0	2.63	0	-6.78
8	0	-1.57	-3.71	0	3.49	0	-4.41

For respondent 4, the difference in preference when moving from $2 to $4 is roughly the same as the preference for a sticker

We can also calculate the relative importance of each attribute for each individual by determining how much each attribute contributes to the overall utility of the choice. We first calculate the range of each

Feature	max(β_i) − min(β_i)	Range	Importance
Price	0 − -3.98	3.98	22.1%
Packaging	0 − -4.24	4.24	23.6%
Flavor	0 − -9.76	9.76	54.3%
Total		17.99	100.0%

	Range			Importance
ID	Price	Packaging	Flavor	Price	Packaging	Flavor
4	3.98	4.24	9.76	22.1%	23.6%	54.3%
5	5.99	0.19	0.14	94.7%	3.1%	2.2%
6	2.87	2.67	8.60	20.3%	18.9%	60.8%
7	3.34	2.63	6.78	26.2%	20.6%	53.2%
8	3.71	3.49	4.41	32.0%	30.0%	38.0%

cor.mlogit(model_mlogit_hierarchical) |> 
  as_tibble(rownames = "coefficient") |> 
  tt() |> 
  format_tt(j = 2:5, fn = scales::label_number(accuracy = 0.001))

coefficient	price$3	price$4	packagingPlastic + sticker	flavorNuts
price$3	1.000	0.899	0.727	0.069
price$4	0.899	1.000	0.529	0.132
packagingPlastic + sticker	0.727	0.529	1.000	0.240
flavorNuts	0.069	0.132	0.240	1.000

4.3 Predictions and transformations

4.3.1 Willingness-to-pay?

4.3.2 Simulated choice shares

Functions from @Feit:2019

predict_mnl <- function(model, data) {
  # Function for predicting shares from a multinomial logit model 
  # model: mlogit object returned by mlogit()
  # data: a data frame containing the set of designs for which you want to 
  #       predict shares. Same format at the data used to estimate model. 
  data.model <- model.matrix(update(model$formula, 0 ~ .), data = data)[ , -1]
  utility <- data.model %*% model$coef
  share <- exp(utility) / sum(exp(utility))
  cbind(share, data)
}

predict_hier_mnl <- function(model, data, nresp =1000) { 
  # Function to predict shares of a hierarchical multinomial logit model
  # model: mlogit object returned by mlogit()
  # data: a data frame containing the set of designs for which you want to
  # predict shares. Same format at the data used to estimate model.
  # Note that this code assumes all model parameters are random
  data.model <- model.matrix(update(model$formula , 0 ~ .), data = data)[ , -1]
  coef.Sigma <- cov.mlogit(model)
  coef.mu <- model$coef[1:dim(coef.Sigma)[1]]
  draws <- MASS::mvrnorm(n = nresp, coef.mu, coef.Sigma)
  shares <- matrix(NA, nrow = nresp, ncol = nrow(data))

  for (i in 1:nresp) {
    utility <- data.model%*%draws[i,]
    share <- exp(utility)/sum(exp(utility))
    shares[i,] <- share 
  }

  cbind(colMeans(shares), data)
}

example_product_mix <- tribble(
  ~price, ~packaging, ~flavor,
  "$2", "Plastic + sticker", "Chocolate",
  "$3", "Plastic + sticker", "Chocolate",
  "$4", "Plastic + sticker", "Chocolate",
  "$2", "Plastic + paper", "Nuts",
  "$3", "Plastic + paper", "Nuts",
  "$4", "Plastic + paper", "Nuts"
) |> 
  mutate(across(everything(), factor))

predict_hier_mnl(model_mlogit_hierarchical, example_product_mix)
##   colMeans(shares) price         packaging    flavor
## 1         0.645363    $2 Plastic + sticker Chocolate
## 2         0.121951    $3 Plastic + sticker Chocolate
## 3         0.045006    $4 Plastic + sticker Chocolate
## 4         0.157910    $2   Plastic + paper      Nuts
## 5         0.021582    $3   Plastic + paper      Nuts
## 6         0.008187    $4   Plastic + paper      Nuts

4.4 Market simulations?