Over time, Chile could increase the voucher for all children, while keeping it progressive, to increase public spending on education. Cross-country empirical evidence suggests that a large share of privately operated schools has a positive impact on PISA results, but only if the share of public financing remains high (WolSmann, 2005). In contrast, school systems with a high share of private financing achieve weaker results than others, presumably because they do not do enough to help children of lower ability catch up. The government oversees only those schools that enter the SEP system and accept the extra subsidy, which has created a disincentive to participate.