Exercise 2: Nested logit model

In mlogit: Multinomial Logit Models

knitr::opts_chunk$set(echo = TRUE, message = FALSE, warning = FALSE, widtht = 65)
options(width = 65)

The data set HC from mlogit contains data in R format on the choice of heating and central cooling system for 250 single-family, newly built houses in California.

The alternatives are:

• Gas central heat with cooling gcc,
• Electric central resistence heat with cooling ecc,
• Electric room resistence heat with cooling erc,
• Electric heat pump, which provides cooling also hpc,
• Gas central heat without cooling gc,
• Electric central resistence heat without cooling ec,
• Electric room resistence heat without cooling er.

Heat pumps necessarily provide both heating and cooling such that heat pump without cooling is not an alternative.

The variables are:

• depvar gives the name of the chosen alternative,
• ich.alt are the installation cost for the heating portion of the system,
• icca is the installation cost for cooling
• och.alt are the operating cost for the heating portion of the system
• occa is the operating cost for cooling
• income is the annual income of the household

Note that the full installation cost of alternative gcc is ich.gcc+icca, and similarly for the operating cost and for the other alternatives with cooling.

@. Run a nested logit model on the data for two nests and one log-sum coefficient that applies to both nests. Note that the model is specified to have the cooling alternatives (gcc},ecc}, erc},hpc}) in one nest and the non-cooling alternatives (gc},ec}, `er}) in another nest.

library("mlogit")
data("HC", package = "mlogit")
HC <- dfidx(HC, varying = c(2:8, 10:16), choice = "depvar")
cooling.modes <- idx(HC, 2) %in% c('gcc', 'ecc', 'erc', 'hpc')
room.modes <- idx(HC, 2) %in% c('erc', 'er')
# installation / operating costs for cooling are constants, 
# only relevant for mixed systems
HC$icca[! cooling.modes] <- 0
HC$occa[! cooling.modes] <- 0
# create income variables for two sets cooling and rooms
HC$inc.cooling <- HC$inc.room <- 0
HC$inc.cooling[cooling.modes] <- HC$income[cooling.modes]
HC$inc.room[room.modes] <- HC$income[room.modes]
# create an intercet for cooling modes
HC$int.cooling <- as.numeric(cooling.modes)
# estimate the model with only one nest elasticity
nl <- mlogit(depvar ~ ich + och +icca + occa + inc.room + inc.cooling + int.cooling | 0, HC,
             nests = list(cooling = c('gcc','ecc','erc','hpc'), 
             other = c('gc', 'ec', 'er')), un.nest.el = TRUE)
summary(nl)

(a) The estimated log-sum coefficient is $0.59$. What does this estimate tell you about the degree of correlation in unobserved factors over alternatives within each nest?

The correlation is approximately $1-0.59=0.41$. It's a moderate correlation.

(b) Test the hypothesis that the log-sum coefficient is 1.0 (the value that it takes for a standard logit model.) Can the hypothesis that the true model is standard logit be rejected?

We can use a t-test of the hypothesis that the log-sum coefficient equal to 1. The t-statistic is :

 (coef(nl)['iv'] - 1) / sqrt(vcov(nl)['iv', 'iv'])

The critical value of t for 95\% confidence is 1.96. So we can reject the hypothesis at 95\% confidence.

We can also use a likelihood ratio test because the multinomial logit is a special case of the nested model.

# First estimate the multinomial logit model
ml <- update(nl, nests = NULL)
lrtest(nl, ml)

Note that the hypothesis is rejected at 95\% confidence, but not at 99\% confidence.

1. Re-estimate the model with the room alternatives in one nest and the central alternatives in another nest. (Note that a heat pump is a central system.)

nl2 <- update(nl, nests = list(central = c('ec', 'ecc', 'gc', 'gcc', 'hpc'), 
                    room = c('er', 'erc')))
summary(nl2)

(a) What does the estimate imply about the substitution patterns across alternatives? Do you think the estimate is plausible?

The log-sum coefficient is over 1. This implies that there is more substitution across nests than within nests. I don't think this is very reasonable, but people can differ on their concepts of what's reasonable.

(b) Is the log-sum coefficient significantly different from 1?

\begin{answer}[5] The t-statistic is :

 (coef(nl2)['iv'] - 1) / sqrt(vcov(nl2)['iv', 'iv'])
lrtest(nl2, ml)

We cannot reject the hypothesis at standard confidence levels.

(c) How does the value of the log-likelihood function compare for this model relative to the model in exercise 1, where the cooling alternatives are in one nest and the heating alternatives in the other nest.

logLik(nl)
logLik(nl2)

The $\ln L$ is worse (more negative.) All in all, this seems like a less appropriate nesting structure.

1. Rerun the model that has the cooling alternatives in one nest and the non-cooling alternatives in the other nest (like for exercise 1), with a separate log-sum coefficient for each nest.

nl3 <- update(nl, un.nest.el = FALSE)

(a) Which nest is estimated to have the higher correlation in unobserved factors? Can you think of a real-world reason for this nest to have a higher correlation?

The correlation in the cooling nest is around 1-0.60 = 0.4 and that for the non-cooling nest is around 1-0.45 = 0.55. So the correlation is higher in the non-cooling nest. Perhaps more variation in comfort when there is no cooling. This variation in comfort is the same for all the non-cooling alternatives.

(b) Are the two log-sum coefficients significantly different from each other? That is, can you reject the hypothesis that the model in exercise 1 is the true model?

We can use a likelihood ratio tests with models nl and nl3.

lrtest(nl, nl3)

The restricted model is the one from exercise 1 that has one log-sum coefficient. The unrestricted model is the one we just estimated. The test statistics is 0.6299. The critical value of chi-squared with 1 degree of freedom is 3.8 at the 95\% confidence level. We therefore cannot reject the hypothesis that the two nests have the same log-sum coefficient.

1. Rewrite the code to allow three nests. For simplicity, estimate only one log-sum coefficient which is applied to all three nests. Estimate a model with alternatives gcc, ecc and erc in a nest, hpc in a nest alone, and alternatives gc, ec and er in a nest. Does this model seem better or worse than the model in exercise 1, which puts alternative hpc in the same nest as alternatives gcc, ecc and erc?

nl4 <- update(nl, nests=list(n1 = c('gcc', 'ecc', 'erc'), n2 = c('hpc'),
                    n3 = c('gc', 'ec', 'er')))
summary(nl4)

The $\ln L$ for this model is $-180.26$, which is lower (more negative) than for the model with two nests, which got $-178.12$.

mlogit documentation built on Oct. 23, 2020, 5:29 p.m.