Chinese chart method of gender prediction, as the one here, can be called fairly accurate. But given large enough sample it proves to be around 50% accurate. That certainly makes sense. The Chinese chart method uses mother’s birth date and age to predict baby’s gender. It completely neglects the father’s contribution.

If we look at the conception from the scientific point of view, chromosomes (genes that carry heredity information) can either be X for female or Y for male. Woman has only X chromosome while man has both X and Y. Gender is determined based on which male chromosome makes a contact with female chromosome. XX is a girl while XY is a boy.

Basically Chinese chart neglects to take this mixture of X and Y chromosomes into account. Being 50% accurate is expected since there is an equal chance of conceiving a boy or a girl.