Does anyone know how Oxfam computes that "the top 1% control fully half of the planet's wealth"? In particular, if someone has more debt than assets does that

Does anyone know how Oxfam computes that "the top 1% control fully half of the planet's wealth"? In particular, if someone has more debt than assets does that count as negative wealth? Negative, in that for the purposes of the calculation it would offset the wealth of others with positive net worth?