If you have your own data, a custom calculator can be created.

The user simpley follows the instructions and enters the name of the marker and the corresponding likelihood ratio.

These are listed and stored on the computer.

This calculator is used to compute the likelihood ratio of a negative or normal ultrasound examination.

The examiner enters the initial risk for trisomy 21, for example, and the likelihood ratio for a normal ultrasound. The new or posterior risk is then computed.

If you need to compute the likelihood for a negative or normal ultrasound, you can do so by filling in the data in this contigency table.