Net Run Rate (NRR) has become the preferred method of breaking ties in multi-team one-day international tournaments.
While it can vary slightly from tournament to tournament, in general a team's net run rate is calculated by deducting from the average runs per over scored by that team, the average runs per over scored against that team. In the event of a team being all out in less than its full quota of overs, the calculation of its net run rate shall be based on the full quota of overs to which it would have been entitled and on the number of overs in which the team was dismissed.
Let's take as an example South Africa's net run-rate in the 1999 World Cup. South Africa's listing in the Group A points table published in the group stages was as follows:
To use this example - South Africa had scored, so far in the tournament, 199 for 9 in 50 overs, 254 for 6 in 47.2 overs and 225 for 7 in 50 overs. Across the three games, South Africa scored 678 runs in a total of 147 overs and two balls (actually 147.333 overs), a rate of 678/147.333 or 4.602 runs per over.
Teams opposing South Africa scored 253 for 5 in 50 overs, 110 all out in 35.2 overs and 103 all out in 41 overs. In the case of the sides bowled out, because they were all out before their allotted 50 overs expired, the run rate is calculated as if they had scored their runs over the full 50 overs.
Therefore, the run-rate scored against South Africa across the first three games is calculated on the basis of 466 runs in a total of 50 + 50 + 50 = 150 overs, a rate of 466/150 or 3.107 runs per over.
The net run-rate is, therefore, the difference between their run-rate for (4.602) and their run-rate against (3.107) which is +1.495.