# 4.7. Criteria for Parametric methods¶

In order to estimate the order of a parametric model, one chose a PSD method such as the `aryule()` function. This function (when given an order) returns a list of AR parameters. The order selected may not be optimal (too low or too high). One tricky question is then to find a criteria to select this order in an optiaml way. Criteria are available and the following example illustrate their usage.

## 4.7.1. Example 1¶

Let us consider a data set (the Marple data already used earlier). We use the aryule function to estimate the AR parameter. This function also returns a parameter called rho. This parameter together with the length of the data and the selected order can be used by criteria functions such as the `AIC()` function to figure out the optimal order.

```
from spectrum import *
from pylab import *
order = arange(1, 25)
rho = [aryule(marple_data, i, norm='biased')[1] for i in order]
plot(order, AIC(len(marple_data), rho, order), label='AIC')
```

The optimal order corresponds to the minimal of the plotted function.

## 4.7.2. Example 2¶

We can look at another example that was look at earlier with a AR(4):

```
import scipy.signal
from spectrum import *
from pylab import *
# Define AR filter coefficients and some data accordingly
a = [1, -2.2137, 2.9403, -2.1697, 0.9606];
x = scipy.signal.lfilter([1], a, randn(1,256))
# study different order
order = arange(1, 25)
rho = [aryule(x[0], i, norm='biased')[1] for i in order]
plot(order, AIC(len(x[0]), rho, order), label='AIC')
```

Here, is appears that an order of 4 (at least) should be used, which correspond indeed to the original choice.