Next: Shocks on exogenous variables, Previous: Auxiliary variables, Up: The Model file [Contents][Index]
For most simulation exercises, it is necessary to provide initial (and possibly terminal) conditions. It is also necessary to provide initial guess values for non-linear solvers. This section describes the statements used for those purposes.
In many contexts (deterministic or stochastic), it is necessary to
compute the steady state of a non-linear model: initval
then
specifies numerical initial values for the non-linear solver. The
command resid
can be used to compute the equation residuals for
the given initial values.
Used in perfect foresight mode, the types of forward-looking models for which Dynare was designed require both initial and terminal conditions. Most often these initial and terminal conditions are static equilibria, but not necessarily.
One typical application is to consider an economy at the equilibrium at time 0,
trigger a shock in first period, and study the trajectory of return to
the initial equilibrium. To do that, one needs initval
and
shocks
(see Shocks on exogenous variables).
Another one is to study, how an economy, starting from arbitrary
initial conditions at time 0 converges toward equilibrium.
In this case models, the command histval
permits to specify different
historical initial values for variables with lags for the
periods before the beginning of the simulation. Due to the design of Dynare,
in this case initval
is used to specify the terminal conditions.
Description
The initval
block has two main purposes: providing guess values
for non-linear solvers in the context of perfect foresight simulations
and providing guess values for steady state computations in both perfect
foresight and stochastic simulations. Depending on the presence of histval
and endval
-blocks it is also used for declaring the initial and
terminal conditions in a perfect foresight simulation exercise.
Because of this interaction of the meaning of an initval
-block
with the presence of histval
and endval
-blocks in perfect foresight
simulations, it is strongly recommended to check that the
constructed oo_.endo_simul
and oo_.exo_simul
variables
contain the desired values after running perfect_foresight_setup
and before running perfect_foresight_solver
. In the presence of leads
and lags, these subfields of the results structure will store the historical
values for the lags in the first
column/row and the terminal values for the leads in the last column/row.
The initval
block is terminated by end;
, and contains lines of the
form:
VARIABLE_NAME = EXPRESSION;
In a deterministic (i.e. perfect foresight) model
First, it will fill both the oo_.endo_simul
and oo_.exo_simul
variables
storing the endogenous and exogenous variables with the values provided by this block.
If there are no other blocks present, it will therefore provide the initial and
terminal conditions for all the endogenous and exogenous variables, because it
will also fill the last column/row of these matrices. For the intermediate simulation periods
it thereby provides the starting values for the solver.
In the presence of a histval
block (and therefore absence of an endval
-block),
this histval
block will provide/overwrite the historical values for the state variables (lags) by
setting the first column/row of oo_.endo_simul
and oo_.exo_simul
.
This implies that the initval
-block in the presence of histval
only sets the terminal values
for the variables with leads and provides initial values for the perfect foresight solver.
Because of these various functions of initval
it is often necessary to provide values for all the
endogenous variables in an initval
block. Initial and terminal conditions are strictly
necessary for lagged/leaded variables, while feasible starting values are required for the solver.
It is important to be aware that if some variables, endogenous or exogenous, are not mentioned in the
initval
block, a zero value is assumed. It is particularly important to keep
this in mind when specifying exogenous variables using varexo
that are not allowed
to take on the value of zero, like e.g. TFP.
Note that if the initval
block is immediately followed by a
steady
command, its semantics are slightly changed.
The steady
command will compute the steady state of the model for all the
endogenous variables, assuming that exogenous variables are kept constant at the value
declared in the initval
block. These steady state values conditional on
the declared exogenous variables are then written into oo_.endo_simul
and take up the
potential roles as historical and terminal conditions as well
as starting values for the solver. An initval
block followed by steady
is therefore formally equivalent to an initval
block with the specified values
for the exogenous variables, and the endogenous variables set to the associated steady state values
conditional on the exogenous variables.
In a stochastic model
The main purpose of initval
is to provide initial guess values
for the non-linear solver in the steady state computation. Note that
if the initval
block is not followed by steady
, the
steady state computation will still be triggered by subsequent
commands (stoch_simul
, estimation
…).
It is not necessary to declare 0
as initial value for exogenous
stochastic variables, since it is the only possible value.
The subsequently computed steady state (not the initial values, use histval for this) will be used as the initial condition at all the periods preceeding the first simulation period for the three possible types of simulations in stochastic mode:
periods
option is specified
To start simulations at a particular set of starting values that are not a computed steady state, use histval.
Options
all_values_required
Issues an error and stops processing the .mod file if there is at least
one endogenous or exogenous variable that has not been set in the initval
block.
Example
initval; c = 1.2; k = 12; x = 1; end; steady;
Description
This block is terminated by end;
, and contains lines of the
form:
VARIABLE_NAME = EXPRESSION;
The endval
block makes only sense in a deterministic model and cannot
be used together with histval
. Similar to the initval
command,
it will fill both the oo_.endo_simul
and oo_.exo_simul
variables
storing the endogenous and exogenous variables with the values provided by this block.
If no initval
-block is present, it will fill the whole matrices, therefore
providing the initial and terminal conditions for all the endogenous and exogenous
variables, because it will also fill the first and last column/row of these matrices. Due to
also filling the intermediate simulation periods it will provide the starting values for the solver as well.
If an initval
-block is present, initval
will provide the historical
values for the variables (if there are states/lags), while endval
will fill
the remainder of the matrices, thereby still providing i) the terminal conditions
for variables entering the model with a lead and ii) the initial guess values
for all endogenous variables at all the simulation dates for the perfect foresight solver.
Note that if some variables, endogenous or exogenous, are NOT mentioned in the
endval
block, the value assumed is that of the last
initval
block or steady
command (if present). Therefore,
in contrast to initval
, omitted variables are not automatically assumed to be 0
in this case. Again, it is strongly recommended to check the
constructed oo_.endo_simul
and oo_.exo_simul
variables
after running perfect_foresight_setup
and before running perfect_foresight_solver
to see whether the desired outcome has been achieved.
Like initval
, if the endval
block is immediately followed by a
steady
command, its semantics are slightly changed.
The steady
command will compute the steady state of the model for all
the endogenous variables, assuming that exogenous variables are kept constant
to the value declared in the endval
block. These steady state values
conditional on the declared exogenous variables are then written into oo_.endo_simul
and therefore take up the potential roles as historical and terminal conditions
as well as starting values for the solver. An endval
block followed by steady
is therefore formally equivalent to an endval
block with the specified values
for the exogenous variables, and the endogenous variables set to the associated steady state values.
Options
all_values_required
See all_values_required.
Example
var c k; varexo x; … initval; c = 1.2; k = 12; x = 1; end; steady; endval; c = 2; k = 20; x = 2; end; steady;
The initial equilibrium is computed by steady
conditional on x=1
,
and the terminal one conditional on x=2
. The initval
-block sets
the initial condition for k
, while the endval
-block sets the terminal
condition for c
. The starting values for the perfect foresight solver are
given by the endval
-block. A detailed explanation follows below the next example.
Example
var c k; varexo x; … model; c + k - aa*x*k(-1)^alph - (1-delt)*k(-1); c^(-gam) - (1+bet)^(-1)*(aa*alph*x(+1)*k^(alph-1) + 1 - delt)*c(+1)^(-gam); end; initval; k = 12; end; endval; c = 2; x = 1.1; end; simul(periods=200);
In this example, the problem is finding the optimal path for consumption and
capital for the periods to , given the path of the exogenous
technology level x
. c
is a forward looking variable and the
exogenous variable x
appears with a lead in the expected return of
physical capital, so we need terminal conditions for them, while k
is a
purely backward-looking (state) variable, so we need an initial condition for
it.
Setting x=1.1
in the endval
-block without a shocks
-block implies that technology
is at in and stays there forever, because endval
is filling all entries of oo_.endo_simul
and oo_.exo_simul
except
for the very first one, which stores the initial conditions and was set to by the initval
-block when not
explicitly specifying a value for it.
Because the law of motion for capital is backward-looking, we need an initial
condition for k
at time . Due to the presence of endval
, this cannot be
done via a histval
-block, but rather must be specified in the initval
-block.
Similarly, because the Euler equation is forward-looking, we need a
terminal condition for c
at , which is specified in the
endval
-block.
As can be seen, it is not necessary to specify c
and x
in the initval
-block and
k
in the endval
-block, because they have no impact on the results. Due to
the optimization problem in the first period being to choose c,k
at given the predetermined capital stock k
inherited from as
well as the current and future values for technology x
, the values for
c
and x
at time play no role. The same applies to the choice of
c,k
at time , which does not depend on k
at . As
the Euler equation shows, that choice only depends on current capital as
well as future consumption c
and technology x
, but not on
future capital k
. The intuitive reason is that those variables are
the consequence of optimization problems taking place in at periods
and , respectively, which are not modeled here.
Example
initval; c = 1.2; k = 12; x = 1; end; endval; c = 2; k = 20; x = 1.1; end;
In this example, initial conditions for the forward-looking variables x
and c
are provided, together with a terminal condition for the backward-looking
variable k
. As shown in the previous example, these values will not affect the simulation
results. Dynare simply takes them as given and basically assumes that there were realizations
of exogenous variables and states that make those choices
equilibrium values (basically initial/terminal conditions
at the unspecified time periods and ).
The above example suggests another way of looking at the use of steady
after initval
and endval
. Instead of saying that the
implicit unspecified conditions before and after the simulation range
have to fit the initial/terminal conditions of the endogenous variables
in those blocks, steady
specifies that those conditions at and
are equal to being at the steady state given the exogenous
variables in the initval
and endval
-blocks. The
endogenous variables at and are then set to the corresponding steady state
equilibrium values.
The fact that c
at and k
at specified in
initval
and endval
are taken as given has an important
implication for plotting the simulated vector for the endogenous
variables, i.e. the rows of oo_.endo_simul
: this vector will
also contain the initial and terminal
conditions and thus is 202 periods long in the example. When you specify
arbitrary values for the initial and terminal conditions for forward- and
backward-looking variables, respectively, these values can be very far
away from the endogenously determined values at and . While the
values at and are unrelated to the dynamics for , they
may result in strange-looking large jumps. In the example above,
consumption will display a large jump from to and capital will
jump from to when using rplot or manually plotting oo_.endo_val
.
Description
In a deterministic perfect foresight context
In models with lags on more than one period, the histval
block
permits to specify different historical initial values for different
periods of the state variables. In this case, the initval
-block takes over the role of specifying
terminal conditions and starting values for the solver. Note that the histval
block does not
take non-state variables.
This block is terminated by end;
, and contains lines of the
form:
VARIABLE_NAME(INTEGER) = EXPRESSION;
EXPRESSION is any valid expression returning a numerical value and can contain already initialized variable names.
By convention in Dynare, period 1 is the first period of the
simulation. Going backward in time, the first period before the start
of the simulation is period 0
, then period -1
, and so on.
State variables not initialized in the histval
block are assumed to
have a value of zero at period 0 and before. Note that histval
cannot be followed by steady
.
Example
model; x=1.5*x(-1)-0.6*x(-2)+epsilon; log(c)=0.5*x+0.5*log(c(+1)); end; histval; x(0)=-1; x(-1)=0.2; end; initval; c=1; x=1; end;
In this example, histval
is used to set the historical conditions for the two lags
of the endogenous variable x
, stored in the first column of oo_.endo_simul
.
The initval
block is used to set the terminal condition for the forward looking variable c
,
stored in the last column of oo_.endo_simul
. Moreover, the initval
block defines
the starting values for the perfect foresight solver for both endogenous variables c
and x
.
In a stochastic simulation context
In the context of stochastic simulations, histval
allows setting
the starting point of those simulations in the state space. As for the case of
perfect foresight simulations, all not explicitly specified variables are set to 0.
Moreover, as only states enter the recursive policy functions, all values specified for control variables will be ignored. This can be used
periods
option is specified. Note that this
only affects the starting point for the simulation, but not for the impulse
response functions. When using the loglinear option, the
histval
-block nevertheless takes the unlogged starting values.
histval
-block nevertheless takes the unlogged starting values.
histval
-block nevertheless takes the unlogged starting values.
Options
all_values_required
See all_values_required.
Example
var x y; varexo e; model; x = y(-1)^alpha*y(-2)^(1-alpha)+e; … end; initval; x = 1; y = 1; e = 0.5; end; steady; histval; y(0) = 1.1; y(-1) = 0.9; end; stoch_simul(periods=100);
This command will display the residuals of the static equations of the
model, using the values given for the endogenous in the last
initval
or endval
block (or the steady state file if you
provided one, see Steady state).
Description
In a deterministic setup, this command is used to specify a path for all endogenous and exogenous variables. The length of these paths must be equal to the number of simulation periods, plus the number of leads and the number of lags of the model (for example, with 50 simulation periods, in a model with 2 lags and 1 lead, the paths must have a length of 53). Note that these paths cover two different things:
The command accepts three file formats:
periods+M_.maximum_lag+M_.maximum_lead
pkg install -forge io
’).
Warning
The extension must be omitted in the command argument. Dynare will automatically figure out the extension and select the appropriate file type.
This command is equivalent to histval
, except that it reads its input
from a file.
This command is typically used in conjunction with smoother2histval
.
Next: Shocks on exogenous variables, Previous: Auxiliary variables, Up: The Model file [Contents][Index]