Between 1992 and 1994, the figure fell by one-fifth.
Between 1994 and 1996, the number dropped by half.
The figure in 1998 was one-tenth the 1992 total.
The USA started at 20% in January before it increased to just under 30% in February, this fell slightly in March and reached only 20% in April before dropping to its lowest point of 10% at the end of May.