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import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import statsmodels.api as sm
%matplotlib inline
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# Import actual and trend production data
data = pd.read_csv('http://www.briancjenkins.com/teaching/winter2017/econ129/data/Econ129_Rbc_Data.csv',index_col=0)
print(data.head())
Recall:
\begin{align} \frac{X_t - X_t^{trend}}{X_t^{trend}} & \approx \log\left(X_t/X_t^{trend}\right) = \log X_t - \log X_t^{trend} \end{align}
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# Create new DataFrame of percent deviations from trend
data_cycles = pd.DataFrame({
'gdp':100*(np.log(data.gdp/data.gdp_trend)),
'consumption':100*(np.log(data.consumption/data.consumption_trend)),
'investment':100*(np.log(data.investment/data.investment_trend)),
'hours':100*(np.log(data.hours/data.hours_trend)),
'capital':100*(np.log(data.capital/data.capital_trend)),
'tfp':100*(np.log(data.tfp/data.tfp_trend)),
})
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# Plot all percent deviations from trend
fig = plt.figure(figsize=(12,8))
ax = fig.add_subplot(3,2,1)
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-',lw=3,alpha=0.7)
ax.grid()
ax.set_title('GDP per capita')
ax.set_ylabel('% dev from trend')
ax = fig.add_subplot(3,2,2)
ax.plot_date(data_cycles.index,data_cycles['consumption'],'-',lw=3,alpha=0.7)
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-k',lw=3,alpha=0.2)
ax.grid()
ax.set_title('Consumption per capita (GDP in light gray)')
ax = fig.add_subplot(3,2,3)
ax.plot_date(data_cycles.index,data_cycles['investment'],'-',lw=3,alpha=0.7)
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-k',lw=3,alpha=0.2)
ax.grid()
ax.set_title('Investment per capita (GDP in light gray)')
ax.set_ylabel('% dev from trend')
ax = fig.add_subplot(3,2,4)
ax.plot_date(data_cycles.index,data_cycles['hours'],'-',lw=3,alpha=0.7)
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-k',lw=3,alpha=0.2)
ax.grid()
ax.set_title('Hours per capita (GDP in light gray)')
ax = fig.add_subplot(3,2,5)
ax.plot_date(data_cycles.index,data_cycles['capital'],'-',lw=3,alpha=0.7)
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-k',lw=3,alpha=0.2)
ax.grid()
ax.set_title('Capital per capita (GDP in light gray)')
ax.set_ylabel('% dev from trend')
ax = fig.add_subplot(3,2,6)
ax.plot_date(data_cycles.index,data_cycles['tfp'],'-',lw=3,alpha=0.7,label='TFP')
ax.plot_date(data_cycles.index,data_cycles['gdp'],'-k',lw=3,alpha=0.2,label='GDP')
ax.grid()
ax.set_title('TFP per capita (GDP in light gray)')
plt.tight_layout()
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# Add a column of lagged tfp values
data_cycles['tfp_lag']= data_cycles['tfp'].shift()
data_cycles = data_cycles.dropna()
data_cycles.head()
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plt.scatter(data_cycles.tfp_lag,data_cycles.tfp,s=50,alpha = 0.7)
plt.grid()
plt.xlabel('TFP lagged one period (% dev from trend)')
plt.ylabel('TFP (% dev from trend)')
Since there appears to be a stong correlation between the lagged cyclical component of TFP and the current cyclical component of TFP, let's estimate the following AR(1) model using the statsmodels package.
\begin{align} \hat{a}_t & = \rho \hat{a}_{t-1} + \epsilon_t \end{align}
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model = sm.OLS(data_cycles.tfp,data_cycles.tfp_lag)
results = model.fit()
print(results.summary())
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# Store the estimated autoregressive parameter
rhoA = results.params['tfp_lag']
# Compute the predicted values:
tfp_pred = results.predict()
# Compute the standard deviation of the residuals of the regression
sigma = np.std(results.resid)
print('rho: ',np.round(rhoA,5))
print('sigma (in percent):',np.round(sigma,5))
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# Scatter plot of data with fitted regression line:
plt.scatter(data_cycles.tfp_lag,data_cycles.tfp,s=50,alpha = 0.7)
plt.plot(data_cycles.tfp_lag,tfp_pred,'r')
plt.grid()
plt.xlabel('TFP lagged one period (% dev from trend)')
plt.ylabel('TFP (% dev from trend)')
Consider the following business cycle model: \begin{align} Y_t & = A_t K_t^{\alpha} \tag{1}\\ C_t & = (1-s)Y_t \tag{2}\\ I_t & = K_{t+1} - ( 1- \delta) \tag{3}\\ Y_t & = C_t + I_t \tag{4} \end{align} where: \begin{align} \log A_{t+1} & = \rho \log A_t + \epsilon_t, \tag{5} \end{align} reflects exogenous fluctuation in TFP. The endogenous variables in the model are $K_t$, $Y_t$, $C_t$, $I_t$, and $A_t$ and $\epsilon_t$ is an exogenous white noise shock process with standard deviation $\sigma$. $K_t$ and $A_t$ are called state variables because their values in period $t$ affect the equilibrium of the model in period $t+1$.
The non-stochastic steady state equilibrium for the model is an equilibrium in which the exogenous shock process $\epsilon_t = 0$ for all $t$ and $K_{t+1} = K_t$ and $A_{t+1} = A_t$ for all $t$. Find the non-stochastic steady state of the model analytically. That is, use pencil and paper to find values for capital $\bar{K}$, output $\bar{Y}$, consumption $\bar{C}$, and investment $\bar{I}$ in terms of the model parameters $\alpha$, $s$, and $\delta$.
Suppose that: $\alpha = 0.35$, $\delta = 0.025$, and $s = 0.1$. Use your answers to the previous exercise to compute numerical values for consumption, output, capital, and investment. Use the variable names kss, yss, css, and iss to store the computed steady state values.
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# Define parameters
# Compute the steady state values of the endogenous variables
In this part, you will simulate the model directly in response to a 1 percent shock to aggregate technology. The simulation will run for $T+1$ periods from $t = 0,\ldots, T$ and the shock arrives at $t = 1$. Suppose that $T = 12$.
Use equations (1) through (4) to solve for $K_{t+1}$, $Y_t$, $C_t$, and $I_t$ in terms of only $K_t$, $a_t$, and the model parameters $\alpha$, $\delta$, and $s$.
Initialize an array for $\epsilon_t$ called eps_ir that is equal to a $\times 1$ array of zeros. Set the first element of this array equal to 0.01.
Initialize an array for $\log A_t$ called log_a_ir that is equal to a $(T+1)\times 1$ array of zeros. Set $\rho = 0.75$ and compute the impulse response of $\log A_t$ to the shock. Use the simulated values for $\log A_t$ to compute $A_t$ and save the values in a variable called a_ir (Note: $A_t = e^{\log A_t}$). Plot $\log A_t$ and $A_t$.
Initialize an array for $K_t$ called k_ir that is a $(T+1)\times 1$ array of zeros. Set the first value in the array equal to steady state capital. Then compute the subsequent values for $K_t$ using the computed values for $A_t$. Plot $K_t$.
Initialize $(T+1)\times 1$ arrays for $Y_t$, $C_t$, and $I_t$ called y_ir, c_ir, and i_ir. Use the computed values for $K_t$ to compute simulated values for $Y_t$, $C_t$, and $I_t$.
Construct a $2\times2$ grid of subplots of the impulse responses of capital, output, consumption, and investment to a one percent shock to aggregate technology.
Compute the percent deviation of each variable from its steady state
\begin{align}
100*(\log(X_t) - \log(\bar{X}))
\end{align}
and store the results in variables called: k_ir_dev, y_ir_dev, c_ir_dev, and i_ir_dev. Construct a $2\times2$ grid of subplots of the impulse responses of capital, output, consumption, and investment to the technology shock with each variable expressed as a percent deviation from steady state.
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# Set number of simulation periods (minus 1):
# Initialize eps_ir as a T x 1 array of zeros and set first value to 0.01
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# Set coefficient of autocorrelation for log A
# Initialize log_a_ir as a (T+1) x 1 array of zeros and compute.
# Plot log_a_ir
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# Computes a_ir.
# Plot a_ir
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# Initialize k_ir as a (T+1) x 1 array of zeros and compute
# Plot k_ir
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# Compute y_ir, c_ir, i_ir
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# Create a 2x2 plot of y_ir, c_ir, i_ir, and k_ir
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# Compute y_ir_dev, c_ir_dev, i_ir_dev, and k_ir_dev to be the log deviations from steady state of the
# respective variables
# Create a 2x2 plot of y_ir_dev, c_ir_dev, i_ir_dev, and k_ir_dev