F Statistics
Computes a series of F statistics for a specified data window.
Fstats(formula, from = 0.15, to = NULL, data = list(), vcov. = NULL)
formula |
a symbolic description for the model to be tested |
from, to |
numeric. If |
data |
an optional data frame containing the variables in the model. By
default the variables are taken from the environment which |
vcov. |
a function to extract the covariance matrix
for the coefficients of a fitted model of class |
For every potential change point in from:to a F statistic (Chow
test statistic) is computed. For this an OLS model is fitted for the
observations before and after the potential change point, i.e. 2k
parameters have to be estimated, and the error sum of squares is computed (ESS).
Another OLS model for all observations with a restricted sum of squares (RSS) is
computed, hence k parameters have to be estimated here. If n is
the number of observations and k the number of regressors in the model,
the formula is:
F = (RSS-ESS)/ESS * (n-2*k)
Note that this statistic has an asymptotic chi-squared distribution with k degrees of freedom and (under the assumption of normality) F/k has an exact F distribution with k and n - 2k degrees of freedom.
Fstats returns an object of class "Fstats", which contains
mainly a time series of F statistics. The function plot has a
method to plot the F statistics or the
corresponding p values; with sctest a
supF-, aveF- or expF-test on structural change can be performed.
Andrews D.W.K. (1993), Tests for parameter instability and structural change with unknown change point, Econometrica, 61, 821-856.
Hansen B. (1992), Tests for parameter instability in regressions with I(1) processes, Journal of Business & Economic Statistics, 10, 321-335.
Hansen B. (1997), Approximate asymptotic p values for structural-change tests, Journal of Business & Economic Statistics, 15, 60-67.
## Nile data with one breakpoint: the annual flows drop in 1898
## because the first Ashwan dam was built
data("Nile")
plot(Nile)
## test the null hypothesis that the annual flow remains constant
## over the years
fs.nile <- Fstats(Nile ~ 1)
plot(fs.nile)
sctest(fs.nile)
## visualize the breakpoint implied by the argmax of the F statistics
plot(Nile)
lines(breakpoints(fs.nile))
## UK Seatbelt data: a SARIMA(1,0,0)(1,0,0)_12 model
## (fitted by OLS) is used and reveals (at least) two
## breakpoints - one in 1973 associated with the oil crisis and
## one in 1983 due to the introduction of compulsory
## wearing of seatbelts in the UK.
data("UKDriverDeaths")
seatbelt <- log10(UKDriverDeaths)
seatbelt <- cbind(seatbelt, lag(seatbelt, k = -1), lag(seatbelt, k = -12))
colnames(seatbelt) <- c("y", "ylag1", "ylag12")
seatbelt <- window(seatbelt, start = c(1970, 1), end = c(1984,12))
plot(seatbelt[,"y"], ylab = expression(log[10](casualties)))
## compute F statistics for potential breakpoints between
## 1971(6) (corresponds to from = 0.1) and 1983(6) (corresponds to
## to = 0.9 = 1 - from, the default)
## compute F statistics
fs <- Fstats(y ~ ylag1 + ylag12, data = seatbelt, from = 0.1)
## this gives the same result
fs <- Fstats(y ~ ylag1 + ylag12, data = seatbelt, from = c(1971, 6),
to = c(1983, 6))
## plot the F statistics
plot(fs, alpha = 0.01)
## plot F statistics with aveF boundary
plot(fs, aveF = TRUE)
## perform the expF test
sctest(fs, type = "expF")Please choose more modern alternatives, such as Google Chrome or Mozilla Firefox.