next up previous
Next: d=2 Ising model Up: No Title Previous: Theory

Gaussian Test Case

In this section, we apply the formalism for the statistical error to the case of a Gaussian energy distribution with independent measurements. While it is clear that no real physical system is described by such a distribution, we nevertheless feel it is instructive to carry out the calculation because it allows us to derive a simple, closed-form expression for the statistical error. In addition, we expect that the behavior of this ``toy'' system will be qualitatively the same as that of a real system.

With independent measurements, all correlation times are zero, and the expression for the relative error (14) is simplified to be:

 

For this example, we will use a continuous probability distribution which is symmetric about E = 0 and has a standard deviation .

The quantities needed for the error analysis are then easily computed by integrating with the appropriate function. For this example, we will calculate the statistical error in the energy and specific heat that the quantities we need are

By inserting these into (15) we find that the expression for the square of the relative error in is

Because the average value of E is zero at , the relative error is not well behaved. Let us therefore consider the absolute statistical error,

 

For , the error reduces to the expected result . For small values of , the error increases moderately, but as gets larger, the error in E begins to increase dramatically due to the exponential term and continues to rise without limit. The error in the specific heat has nearly the same form as the error in the energy:

 

While the explicit form will be different for real systems, the qualitative behavior of the error should remain: For a region around , the errors will increase only moderately, then will begin to rise rapidly as increases.



next up previous
Next: d=2 Ising model Up: No Title Previous: Theory



root
Thu Jun 22 14:26:19 EDT 1995