# Sensitivity analysis for a piston model¶

So you have some data, and you’d like to get a “best-fit” curve through the data. No problem. That’s pretty much what this guide seeks to do. So let’s get started; consider the data set below.

Sample data for regression.

X

Y

0

6.8053

0.0714

-1.5184

0.1429

1.6416

0.2857

6.3543

0.3571

14.3442

0.4286

16.4426

0.5714

18.1953

0.6429

28.9913

0.7143

27.2246

0.7857

40.3759

0.9286

55.3726

1.0000

72.0

Our goal is to compute the best fit a polynomial approximation through this data set. We begin by calling two classes from the code. The first class is the Parameter class, that handles all the information regarding the independent variable, or parameter. The second class is the Polyreg class, which actually generates the “best-fit” curve using the information in the parameter. To begin, lets load the data.

```from equadratures import *
import numpy as np

dimensions = 1
M = 12
param = Parameter(distribution='Uniform', lower=0, upper=1., order=1)
myParameters = [param for i in range(dimensions)] # one-line for loop for parameters
x_train = np.mat([0,0.0714,0.1429,0.2857,0.3571,0.4286,0.5714,0.6429,0.7143,0.7857,0.9286,1.0000], dtype='float64')
y_train = np.mat([6.8053,-1.5184,1.6416,6.3543,14.3442,16.4426,18.1953,28.9913,27.2246,40.3759,55.3726,72.0], dtype='float64')
x_train = np.reshape(x_train, (M, 1))
y_train = np.reshape(y_train, (M, 1))
```

Now we use the univariate basis in the polyreg class.

```myBasis = Basis('Univariate')
poly = Poly(myParameters, myBasis, method='least-squares', sampling_args={'sample-points':x_train, 'sample-outputs':y_train} )
poly.set_model()
N = 100
x_test = np.reshape(np.linspace(0, 1, N), (N, 1) )

# Plot the results
fig = plt.figure()
for i in range(0, M):
plt.scatter(x_train[i,0], y_train[i,0], marker='o', s=80, color='tomato')
plt.plot(x_test, poly.get_polyfit(x_test), 'k-')
plt.xlabel('\$X\$', fontsize=13)
plt.ylabel('\$Y\$', fontsize=13)
plt.xticks(fontsize=13)
plt.yticks(fontsize=13)
``` Figure. A linear model fit (-) for the data (o).

Now, we repeat the same experiment, but increase the order of the polynomial!

```myBasis = Basis('Univariate')
param = Parameter(distribution='Uniform', lower=0, upper=1., order=2)
myParameters = [param for i in range(dimensions)] # one-line for loop for parameters
poly = Poly(myParameters, myBasis, method='least-squares', sampling_args={'sample-points':x_train, 'sample-outputs':y_train} )
``` Figure. A quadratic model fit (-) for the data (o).

The full source code for this tutorial can be found here.