################## Practice Exam # 1 (Code) ##################

FMX_da <- read.csv("http://www.bauer.uh.edu/rsusmel/4397/FX_USA_MX.csv", head=TRUE, sep=",")
summary(FMX_da)

x_years <- FMX_da$Years
us_CPI <- FMX_da$US_CPI
us_M1 <- FMX_da$US_M1
us_i <- FMX_da$US_int
us_GDP <- FMX_da$US_GDP
us_CA <- FMX_da$US_CA
mx_CPI <- FMX_da$MX_CPI
mx_M1 <- FMX_da$MX_M1
mx_i <- FMX_da$MX_int
mx_GDP <- FMX_da$MX_GDP
mx_CA <- FMX_da$MX_CA
S_mx <- FMX_da$MXN_USD

T <- length(us_CPI)
us_I <- log(us_CPI[-1]/us_CPI[-T])	# US Inflation
us_y <- log(us_GDP[-1]/us_GDP[-T])	# US growth rate
us_ca <- us_CA[-1]/us_CA[-T] - 1	# US CA % change
us_mg <- log(us_M1[-1]/us_M1[-T])	# US Money supply growth rate
mx_I <- log(mx_CPI[-1]/mx_CPI[-T])	# Mexican Inflation
mx_y <- log(mx_GDP[-1]/mx_GDP[-T])	# Mexican growth rate
mx_ca <- mx_CA[-1]/mx_CA[-T] - 1	# Mexican CA % change
mx_mg <- log(mx_M1[-1]/mx_M1[-T])	# Mexican Money supply growth rate
e_mx <- log(S_mx[-1]/S_mx[-T])		# Exchange rate MX Pesos (MXN) per USD or (MXN/USD)
us_i_1 <- us_i[-1]/100			# US interest rate (at time t)
mx_i_1 <- mx_i[-1]/100			# MX interest rate (at time t)
mx_i_0 <- mx_i[-T]/100			# Lagged MX interest rate (at time t-1)

T_new <- length(mx_i_1)


###### Q1 ###### 

x <- us_y

### Q1.a. Histogram
n_breaks <- 40
h <- hist(x, breaks=n_breaks, col="red", xlab="% US Growth Rate",  main = "Distribution - Histogram for US growth")


### Q1.b. Moments for us_y
m1 <- sum(x)/T         ## Mean
T <-length(x)
m1
m2 <- sum((x-m1)^2)/(T-1)  ## Var
sd <- sqrt(m2)	     ## SD
sd
m3 <- sum((x-m1)^3)/T  ## For numerator of S
m4 <- sum((x-m1)^4)/T  	## For numerator of K
b1 <- m3/m2^(3/2)  	## Sample Skewness
b1
b2 <- (m4/m2^2)        ## Sample Kurtosis
b2


### Q1.c. Test for Normality of e_mx 
JB <- (b1^2+(b2-3)^2/4)*T/6
JB
qchisq(.95, df = 2) 	# chi-squared (df=2) value at 5% level
p_val <- 1 - pchisq(JB, df = 2)  	# p-value of LM_test 
p_val


### Q1.d. Test for mean(us_y) = 0
t_us_y <- m1/(sd/sqrt(T))
t_us_y


### Q1.e. 95% C.I for SD(us_y) - Using asymptotic distribution (CLT)
SE_s <- sd/sqrt(2*(T-1))
us_y_sd_ci <- c(sd - 1.96*SE_s, sd + 1.96*SE_s)

# 95% C.I for SD(us_y) - Using a bootstrap
library(boot)
sim_size <- 1000

# function to obtain the SD from the data
sd_p <- function(data, i) {
  d <- data[i]
  return(sd(d))
}

boot.samps <- boot(data=x, statistic=sd_p, R=sim_size)	# resampling and theta* estimation

# Percentile method using boot
boot.ci(boot.samps, type = "basic")		


### Q1.f. Test if annual mean = 3%
t_us_y_a <- (m1*4 - .03)/(sd * sqrt(4)/sqrt(T/4))
t_us_y_a	



###### Q2 ###### 
### Size 100
sim_size = 100

dat_i <- data.frame(mx_I, mx_i_1)

library(boot)

# function to obtain cor from the data
cor_xy <- function(data, i) { 
	d <-data[i,]
  	return(cor(d$mx_I,d$mx_i_1))
}

# bootstrapping with sim_size replications
boot.samps <- boot(data=dat_i, statistic=cor_xy,   R=sim_size)


## Q2.a. view stored bootstrap samples and compute mean
boot.samps
mean(boot.samps$t)		# our estimate of the correlation
sd(boot.samps$t)		# SD of the correlation estimate


### Q2.b. Size 1,000
sim_size = 1000
# bootstrapping with sim_size replications
boot.samps <- boot(data=dat_i, statistic=cor_xy,   R=sim_size)

# view stored bootstrap samples and compute mean
boot.samps
mean(boot.samps$t)		# our estimate of the correlation
sd(boot.samps$t)		# SD of the correlation estimate


### Q2.c. 95% CI using percentile method
 boot.ci(boot.samps, type="perc")


### Q2.d. 95% C.I for MX Interest Rates using sampling distribution, asumming bivariate normality for data (mx_I, mx_i_1)
T_n <- T-1					# sample size
m_cor <- cor(mx_I, mx_i_1)
sd_cor <- sqrt(1-m_cor^2)/sqrt(T_new-2)

cor_lb <- m_cor - 1.96 * sd_cor 
cor_lb
cor_ub <- m_cor + 1.96 * sd_cor  
cor_ub


### Q2.e. Real Interest Rates in MX - H0: Mean = 0
m_disc <- mx_i_1 - mx_I
m1_ri_mx <- mean(m_disc)
sd_ri_mx <- sd(m_disc)
t_e_ri_mx <- mean(m1_ri_mx)/(sd_ri_mx/sqrt(T))
t_e_ri_mx
pval <- 1 - pnorm(t_e_ri_mx)
pval



###### Q3 ###### 
## Regression 
fit_i <- lm(mx_i_1 ~ us_i_1 + e_mx + mx_y + mx_ca)		# Model (US int, changes in S, MX gdp growth, MX CA % changes)

### Q3.a. Report Regression
summary(fit_i)


### Q3.b. Interpret US_int coefficient and t-value


### Q3.c. Drivers (significant variables)


### Q3.d. Interpret R^2 and report F-goodness of fit test


### Q3.e. Test if beta_1 = 1
b_i <- fit_i$coefficients			# Extract from lm function OLS coefficients
SE_i <- sqrt(diag(vcov(fit_i)))			# SE from fit_i (also a kx1 vector)
t_beta1 <- (b_i[2] - 1)/SE_i[2]			# t-stat for H0: Beta1 - 1
p_val_beta1 <- (1- pnorm(abs(t_beta1))) * 2	# pvalue for t_beta
p_val_beta1


### Q3.f. F-test for beta_2 = beta_4 = 0
# using library lmtest
library(lmtest)
fit_i_r <- lm(mx_i_1 ~ us_i_1 + mx_y )	
waldtest(fit_i, fit_i_r)			# fit_i is the U Model; fit_i_r is the R Model

## You can get the same answer by computing the F test with the restricted and unrestricted RSS.
#  Unrestricted Model RSS
e_i <- fit_i$residuals				# Unrestricted residuals
RSS <- sum(e_i^2)				# Unrestricted RSS

#  Restricted Model RSS
e_i_r <- fit_i_r$residuals
RSS_r <- sum(e_i_r^2)				# Restricted RSS
k <- length(fit_i$coefficients)			# k: number of coefficients in U Model
k2 <- length(fit_i_r$coefficients)		# k2: number of coefficients in R Model

#  F-test
J <- k - k2					# J: number of restrictions (= k - k2)
T <- length(e_i_r)
F_test <- ((RSS_r - RSS)/J)/(RSS/(T-k))
F_test
qf(.95, df1=J, df2=(T-k)) 
p_val <- 1 - pf(F_test, df1=J, df2=(T-k))
p_val


### Q3.g. Testing J Hypothesis with R package car (J=2, beta_1 = 1 & beta_3 = 2)
library(car)
linearHypothesis(fit_i, c("us_i_1 = 1","mx_y = 2"), test="F") 	# F€: exact test 


### Q3.h. Wald test for missing US Inflation in model (using library lmtest, called above)
fit_i_I <- lm(mx_i_1 ~ us_i_1 + e_mx + mx_y + mx_ca + us_I)	# Now, U Model
waldtest(fit_i_I, fit_i)					# fit_i is the R Model

summary(fit_i_I)				# We could have done a t-test on us_I		

## (EXTRA) Wald test for missing MX Inflation in model (using library lmtest)
fit_i_Im <- lm(mx_i_1 ~ us_i_1 + e_mx + mx_y + mx_ca + mx_I)	#Now, U Model
waldtest(fit_i_Im, fit_i)

summary(fit_i_Im)				# We could have done a t-test on mx_I


### Q3.i. Structural Change  with Chow Test
# Compute Chow test with R package strucchange
library(strucchange)
x_years[68]					# Check Tequila effect date
x_break <- 67					# We lost one observation when taking log changes
sctest(mx_i_1 ~ us_i_1 + e_mx + mx_y + mx_ca, type = "Chow", point = x_break)


### Q3.j. Expected Mexican interest rate according to Model
Ex_Mx_i <- b_i[1] + b_i[2] * mean(us_i_1) + b_i[3] * mean(e_mx) + b_i[4] * mean(mx_y) + b_i[5] * mean(mx_ca)

mean(mx_i)/100			# Observed Mexican interest rate in the period

# Compute Expected Mexican interest rate with fitted values
mx_i_hat <- fit_i$fitted
mean(mx_i_hat)

## (EXTRA) Plot fitted values vs actual values
plot(mx_i_hat, type="l", main="Plot: Fitted Values",
   xlab="time ", ylab="Fitted values ", pch=19)
lines(mx_i_1, type="l", col="red")



###### Q4 ###### (NOT COVERED)
fit_M1 <- lm(y ~ us_i_1 + e_mx + mx_I + mx_y)
y_hat1  <- fitted(fit_M1)
fit_J1 <- lm( y ~ mx_i_0 + us_I + y_hat1)
summary(fit_J1)

fit_M2 <- lm(y ~ mx_i_0 + us_I)
y_hat2  <- fitted(fit_M2)
fit_J2 <- lm( y ~ us_i_1 + e_mx + mx_I + mx_y + y_hat2)
summary(fit_J2)

fit_enc <- lm( y ~ us_i_1 + e_mx + mx_I + mx_y + mx_i_0 + us_I)
summary(fit_enc)

library(lmtest)
jtest(fit_M1, fit_M2)
encomptest(fit_M1,fit_M2)



###### Q5 ###### True of False 
# a. A Restricted model can have a lower RSS than an unrestricted model. // False
# b. Joint Hypothesis and individual hypothesis do not have to reach same conclusions. // True
# c. Type I error can be greater than Type II error. // True
# d. Perfect multicollinearity is not a problem for the unbiasedness of the OLS regression. // False. 
# e. The sampling distribution of the mean drives the sampling distribution of the t-tests and Wald-tests in small (finite) samples. // True
# f. The J-test can be used to test restrictions of the model. // False (NOT COVERED)
# g. If the model is misspecified, OLS is not longer unbiased and consistent. // True (NOT COVERED)
# h. The Chow test can be used when three or more regimes are suspected in the data. // False
# i. If returns are not normally distributed, it is not possible to test the CAPM. // False