This does not mean, of course, that net migration has not fallen, just that, according to some experts, the government's main method of measuring its progress is little better than a rough guess.
While the CPI at one time was the measure desired by the public, government efforts turned the CPI away from measuring the price changes in a fixed-weight basket of goods and services to a quasi-substitution-based basket of goods, which destroyed the concept of the CPI as a measure of the cost of living of maintaining a constant standard of living.