@Override
    public double sd() {
        return Math.sqrt(1 - p) / p;
    }
@Test
    public void testSd() {
        System.out.println("sd");
        ShiftedGeometricDistribution instance = new ShiftedGeometricDistribution(0.3);
        instance.rand();
        assertEquals(2.788867, instance.sd(), 1E-6);
    }